It's a starting point, not the be-all, end-all of evidence. If you lacked a Real ID, and were subsequently found to be in the US illegally, then prosecutors have evidence of that crime and you can be tried for illegal entry and if found guilty, deported.
Many illegals do have American bank accounts. Many make wire transfers out of the country. Whether an illegal is flying or driving, they need identification on them to do so. If stopped for a traffic offense, you have to produce a driver's license along with insurance and vehicle registration. You won't or can't, you're likely going to jail as a result of the stop. That's as true of idiot Sovereign citizens as it legal residents, and it equally applies to illegals in the US.