I have taken courses in both disciplines, and taught students who have come out of both processes, and Classical students almost always have better Greek than NT students. There’s a very simple set of reasons for this though, and it doesn’t really have to do with NT students only doing Koine.
Classical students generally train in classical grammar, which on the whole is slightly more complex than Koine grammar. That’s just the reality of the process of koinification going on in the history of the Greek language.
They then move on to reading fairly demanding literature: Plato, Homer, Greek plays, Attic oratory. These are all high-level literary texts that demand a lot from their students. It’s functionally equivalent to taking a grammar class in English and then in your second year reading Shakespeare and the like.
Thirdly, depending on how they acquire their Greek, they are very likely in a multi-year program that involves reading a considerable amount of such Greek. If they’re a classics major, this is over 50, and up to 100% of their course-load for several years.
Seminary students, though, move from a a grammar class to reading the newspaper. The NT is not a high-register set of documents. It’s sophisticated, challenging, but its level of language isn’t and isn’t meant to be ‘Shakespearean’.
Secondly, they read a fairly closed corpus. Seminary students very rarely read outside the NT at all. In fact, it’s quite possible for them to get through Masters degrees without reading outside the NT. Only the few who are outliers, for whatever reason, will read outside that closed corpus, and even then it can be quite a limited foray.
Thirdly, working in Greek texts amounts to a small amount of most seminary programs. It would rarely rise about 25% of the course load, and overall may be somewhere closer to 10% of a whole degree program. That would only really change if there was some NT specialisation going on.
No one should be surprised then that seminary students come off poorly against students trained in Classical greek. It has very little to do with ‘rigour’ or approach, and mostly to do with sheer time and volume of material, and secondarily type of literature read, which is bleedingly obvious. Should seminary students ‘do more Greek’ and ‘read more widely’? Well, it wouldn’t be a bad thing, but most seminary students are training to be NT scholars, and just piling on more Greek has to mean less of other things.
There’s no punch-line to this post. No agenda or solution or suggestion. I’m just observing that this is the way things are and it’s entirely reasonable that things are this way.