As far as the spurious nature of some sources, it is also not uncommon for earlier authors (Durham, Gillespie, Bownd) before the cases were made or widely known against them, to cite what are now known as spurious or miss-attributed works as the basis of arguments. In other words, I wouldn't knock Turretin too hard on this, unless the case had been made and widely known by his time against Sinensis's sources.
Sure, I'm not one to just ad hominem disregard other things an obviously capable person wrote because they blundered in certain matters.
Still, I find it curious that the stated complaint was that Latin translators supposedly disregarded what they found in Greek manuscripts - but isn't it the case that virtually all of the extant examples of the Comma are in fact contained in Latin manuscripts, while it is absent from virtually all extant Greek manuscripts? So unless it were supposed most of "all the Greek" manuscripts asserted as evidence were lost between the 4th century (Jerome/pseudo-Jerome) and 17th century (Turretin)--which I've never seen proposed--you would think its relative absence in the Greek would be a somewhat known convention among third generation Reformed scholars, and a relatively easy issue to verify.
In terms of the collection containing Hyginus' letter, which accompanied the extremely notorious Donation of Constantine, it seems these writings were widely believed to have been a terrific hoax beginning in the 15th century - as was first demonstrated by a RC scholar! For sure, some Catholics were slow to give the collection up as spurious (ala, obviously, Sinensis), but Protestants were often quite eager to point this out to their Catholic antagonists.
Admittedly, I don't know enough about Turretin's circumstances to make a full assessment, but it does seem he may have been hasty in the matter, at least in accepting Sinensis' stated arguments.