In dealing with certain aspects of relativity, like inertial reference frames, it is accurate to say that there is no preferred reference frame for the universe. There is no aether, no “true” zero velocity. Every inertial reference frame is equally valid; if you are in a spaceship passing me at 0.5c, then you can define a reference frame such that you are actually at rest and I am actually passing you at 0.5c.
But there is a difference between inertial and non-inertial reference frames. A non-inertial reference frame defines “zero velocity” within a rotating or accelerating system. The most common example of this would be centrifugal force: the apparent force pushing you to the edge of a rotating body (like a car navigating a turn). Another example would be the Coriolis force, which causes hurricanes to rotate and deflects ballistic missiles launched between different latitudes. Non-inertial reference frames can be extremely useful (for example, you can define a pool table as being “at rest” on a rotating spherical Earth and therefore derive the fictitious forces that would alter the path of the cue), but they are not “real” in any meaningful sense. They are a knowing and deliberate departure from inertial reference frames, used to simplify an n-body problem and achieve a particular outcome.
Forces in inertial reference frames are real; forces in non-inertial reference frames can be real or fictitious.
Synchrony conventions are like reference frames: they are ways of measuring reality. They are mathematical tools to help simplify equations. An anisotropic synchrony convention, like a non-inertial reference frame, can be a useful tool for solving certain problems. For example, astronomers use a de facto anisotropic synchrony convention in nomenclature for supernovae, identifying their age and evolution based on the date their light reached Earth (1987A, etc.) rather than the YBP when they actually exploded.
But, like a non-inertial reference frame, an anisotropic synchrony convention is a deliberate departure from the most accurate modeling of reality, used to simplify the math. They produce fictitious results. Lisle is not incorrect in stating that they are “only a convention” but I do believe he misleads, to some degree, by implying that an anisotropic synchrony convention is just as representative of reality as an isotropic one.
His “model” is that the Bible is using an anisotropic synchrony convention and creation on the “4th day” is given within an anisotropic nomenclature.
There is a very good reason to choose an isotropic convention, although one which would likely escape Lisle. If you’ll recall…our whole universe was in a hot, dense state, then nearly 14 billion years ago, expansion started. We know from the CMB that there was contact between all parts of the observable universe during the pre-inflationary epoch. This means that we started at a point in which all parts of the universe had essentially the same location, making an anisotropic convention impossible. We define comoving time relative to the Big Bang because that is how we started.