If the answer to the question is no (i.e. you can’t coherently apply the EHT analysis to non-black hole objects), then at some level the people fitting data to the BH image to find M are verifying a prediction of GR, even if they are not the ones actually doing the predicting. Maybe it’s more accurate to say the prediction was Einstein’s, and the EHT people @PdotdQ mentioned are mostly working out the implications of assumptions in GR and seeing how they fit to the data.
That being said, I agree with @PdotdQ’s original comment, which is that there are a lot of scientists (physicists and astronomers) who don’t really make predictions, but instead make theory-laden measurements. In Kuhnian terms, they are scientists working within the widely accepted scientific paradigm. In fact, this describes the vast majority of the daily work of scientists. In fact I would describe my own experiment, measuring the EDM of an electron, as doing just that: we are using established techniques, refined and customized for this particular species of molecule and experimental setup, in order to measure a property of the electron based on theoretical assumptions that everyone agrees upon.
(Of course, the reason that this measurement is interesting is that there are people who have made predictions about the value of the electron EDM, but those are mostly particle theorists, none of whom actually do the experimental work, which is based on “regular” atomic physics. Secondly, just as I said for the EHT case, one could say that the fact that we are able to successfully use basic atomic physics to take and fit data in a coherent manner to our daily experimental results is a testament to the explanatory power of atomic physics and quantum mechanics.)
Chronological Ordering and Evidential Status
Overall, I’m not sure that chronological ordering of the evidence is always relevant for describing the epistemic status of a theory. This has been an area of scholarship in contemporary philosophy of science. For example, Stephen Brush in Prediction and Theory Evaluation: The Case of Light Bending argues that the successful prediction of a new phenomenon is not as convincing as an explanation of past facts, “at least until competing theories have had a chance (and failed) to explain it”. In particular he uses the example of the historical verification of GR. After Einstein devised GR, he found it to be able to explain the advance of Mercury’s perihelion, a previously known fact. GR was able to precisely predict what Newtonian mechanics had failed to do. On the other hand, GR also predicted the phenomenon of light-bending, which Eddington famously confirmed via his observations in 1919. Now if one believes that chronological ordering is important, one could imagine that Eddington’s discovery should have been viewed as a stronger piece of evidence for GR than the post-diction of explaining Mercury’s perihelion. But according to Brush, the historical documents and scientific correspondence of the time show us that this was not necessarily the case:
It later became clear to the experts that the Mercury orbit was stronger evidence for general relativity than light bending. In part this was because the observational data were more accurate it was very difficult to make good eclipse measurements, even with modern technology (36, 37, 71) - and in part because the Mercury orbit calculation depended on a “deeper” part of the theory itself (36, 72) . The fact that light bending was a forecast whereas the Mercury orbit was not seems to count for little or nothing in these judgments…
But the most significant argument (though it was not often explicitly stated) is that, rather than light bending providing better evidence because it was predicted before the observation, it actually provides less secure evidence for that very reason. This is the case at least in the years immediately following the announcement of the edipse result, because scientists recognized that any given empirical result might be explained by more than one theory. Because the Mercury orbit discrepancy had been known for several decades, theorists had already had ample opportunity to explain it from Newtonian celestial mechanics and had failed to do so except by making implausible ad hoc assumptions (33). This made Einstein’s success all the more impressive and made it seem quite unlikely that anyone else would subsequently come up with a better alternative explanation. Light bending on the other hand, had not previously been discussed theoretically (with rare exceptions), but now that the phenomenon was known to exist one might expect that another equally or more satisfactory explanation would be found (74).
Brush goes on to explain that only once other competing theories had failed to explain light bending (about 10 years after the experiment), did light bending become just as important a piece of evidence for GR as Mercury’s orbit. Thus, in Brush’s view, what’s important isn’t the chronological order of the evidence, but how well the theory can explain all the evidence we have, regardless of their chronology.