AR-15s

Nope. Some parts are close.
But nope. They advertise it as such, but it's a marketing term and not reality.
The MIL drawings are very specific and few places actually adhere to them. Much like aftermarket car parts - they look like the OEM stuff, yet suck in comparison.
"Close enough" gets thrown around so much it's a joke in the industry. Then guns gack and the manufacturer just shrugs their shoulders "should have been fine".

This is a useless argument because eventually we will be arguing the definition of “mil spec” I’ve lost count of how many rifles I’ve assembled and I still have a stack of lowers to complete. All are advertised as mil spec and I have yet to have a single instance of a mil spec part not fitting a rifle.