I was about to assert the same as you with as much confidence, but the etymology source I trust most (EtymOnline) nearly agrees with OP [0]:
> 1763, in reference to tables, charts, etc., "pertaining to or forming a synopsis," from Modern Latin synopticus, from Late Latin synopsis (see synopsis). It was being used specifically of weather charts by 1808. Greek synoptikos meant "taking a general or comprehensive view."
> The English sense "affording a general view of a whole" emerged by mid-19c. The word was used from 1841 specifically of the first three Gospels, on notion of "giving an account of events from the same point of view." Related Synoptical (1660s). The writers of Matthew, Mark, and Luke are synoptists.
The subtle change vs OP's is that EtymOnline does include some sense that the word 'synoptic' should be understood to describe the way in which the works relate to one another. But they do say that the connection to 'synopsis' is, in fact, part of the original intent of the usage.
