While I don’t think Moreland’s definition of scientism fully captures the picture, neither does Moran’s account of what science is. There’s a long history behind these accusations of scientism. Part of it is tied into logical positivism, which dominated philosophy in the early to mid-20th century. But another part of it is tied to the writings of New Atheists (early 21st century). Dawkins, Harris, and others basically defined that knowledge is only valid if obtained using definition b), but they also tend to believe that only the hard natural sciences - biology, chemistry, physics and others - use the scientific method properly, as defined by b).
Unsurprisingly, the New Atheists had a low opinion of theology. But in the early days, some New Atheists were also notoriously suspicious of the utility of philosophy as a discipline (not Christian philosophy, but philosophy in general), believing that natural science is all you need. Here is Sam Harris in 2010:
First, a disclaimer and non-apology: Many of my critics fault me for not engaging more directly with the academic literature on moral philosophy. There are two reasons why I haven’t done this: First, while I have read a fair amount of this literature, I did not arrive at my position on the relationship between human values and the rest of human knowledge by reading the work of moral philosophers; I came to it by considering the logical implications of our making continued progress in the sciences of mind. Second, I am convinced that every appearance of terms like “metaethics,” “deontology,” “noncognitivism,” “anti-realism,” “emotivism,” and the like, directly increases the amount of boredom in the universe.
Or for an even more facepalm worthy opinion, here is Bill Nye on philosophy:
Stephen Hawking also infamously said in his book The Grand Design that “philosophy is dead.” Now, I don’t know what Harris and Nye’s opinions are on the subjects of English literature, history, cultural studies, religious studies, or musicology, but I would suspect that it is also not nearly as high as their opinion on utility of the hard sciences. The point is that scientism is an epistemology which asserts that all knowledge must be based on strict, repeatable, systematic empirical observation - whether it be in the sciences or humanities. A belief can only be rational if it is based on such an epistemology. (This is why people like @John_Harshman are adamant that even history is based on the scientific method.)
This is what many theists object to - an arrogant epistemology that exalts the natural sciences as the paradigmatic way of knowing, and subsumes all other subjects under the umbrella of the scientific method. Of course, one reason theists object to this is that most would agree that their belief in God is not based on scientific evidence, but other kinds of evidence.