GREASE means that when you build your "advanced" security device which "protects" customers by treating everything different as hostile, it won't work, so you'll need to tweak it to at least just ignore such differences as irrelevant, which is enough that we can come back later and intentionally improve things.

Previously, without GREASE, we'd have to guess what oversights we could exploit to avoid this "protection" to deliver protocol improvements, if we guessed wrong nothing works, or everybody's security is broken, sometimes both. e.g. for TLS 1.2 the oversight we found is, if you're resuming an existing session the "security" products just wave that through because otherwise they break people's real workflows. So in TLS 1.3 protocol as actually spoken essentially an initial connection goes like this:

Client: Hi some.dns.name.example I'm a TLS 1.2 Client, I'd like to resume our previous conversation #randomNonsense. Also, completely unrelated, I happen to speak FlyCasualThisIsReallyTLSv1.3 and so I have these TLSv1.3KeyAgreementParameters.

then either:

TLS 1.3 Server: Hi Client, of course, let's continue from there. [ whereupon everything further is encrypted because this is actually TLS 1.3, but to a dumb middlebox it makes sense that they're just resuming a prior encrypted conversation #randomNonsense which it doesn't remember ]

OR

TLS 1.2 Server: Er, no I don't remember any such conversation and I don't know FlyCasualThisIsReallyTLSv1.3 so let's start a fresh conversation as normal.

If you are, good on ya. Continue doing $deity's/Bob's work.