I've been a fairly serious flight simmer for about 20 years now, including 13 years of DCS, and have flown the DCS F-18 since it's initial release 4 years ago. This topic gets discussed a fair bit within the flight sim community, and we mostly conclude that we'd likely get the F-18 into the air, but would most likely kill ourselves: either passing out from lack of tolerance and training to handle the G forces, lack of familiarity with the sensation of flight which can wreak havoc on your inner ear and result in vertigo, dizzyness, nausea, or paying attention to any of the hundreds of small details and checks that real pilots do that you don't do in DCS (is the OBOGS working properly? cabin pressurization working properly, icing, etc). Simulator pilots would also not likely be able to handle any inflight emergency or problem in the air. Then, assuming we didn't already kill ourselves during the flight, we'd at best damage the aircraft during the landing, or kill ourselves and destroy the aircraft at worst.
Still, DCS offers a tremendous value as a low cost training platform. The DCS A-10C module was built for the US Air National Guard to use as a training simulator platform to train A-10C pilots, and other countries and airforces are increasingly using DCS to train their pilots. A Spanish company built the Aviojet C-101 module for DCS because it is used in the Spanish airforce and they wanted to use DCS as a training platform. A Chinese company built the JF-17 module for DCS. An Italian company is currently building an MB-339 module for DCS.
You can search online and find images of Chinese fighter pilots using DCS for training. There are a ton of things you can train to in DCS very cost effectively - practicing communications, tactical formations, administrative tasks and procedures, weapon switchology, etc. It doesn't completely replace real flight training of course, but it sure can help countries and militiaries with limited budgets stretch their training budgets.
> I also wonder how come most of this isn't classified
All of the "good stuff" is very, very classified. Particularly electronic warfare, radar performance, modern beyond visible range tactics, modern weapons performance porifiles, nuclear weapons delivery profiles.
A lot of the "nuts and bolts stuff" and basic training materials is unclassified and readily available. If you read through and study all of these documents you'll be well on your way to being a fairly competent virtual fighter pilot:
https://www.cnatra.navy.mil/pubs-pat-pubs.asp
Most of the topics discussed there are fairly "traditional" fighter pilot stuff that have been discussed by airforces for over a hundred years now, so aren't really secret, even though they're being flown in a modern jet trainer like the T-45.