What browser incompatibilities do exist are case-by-case basis, not all at once, depending on business domain and solution design, and even if, like you point out, you can just carpet bomb the issue with a poly/ponyfill.
The infantilization in frontend is accelerated by its veneer of supporting Product's sole mission in life to ship products ASAP, and I fucking hate it, and I've been some frontend engineer somehwere unimportant for 5 years now.
I wager, generalizing from my quite limitde experience, that a concerning amount of frontend devs just know how to schlep bags of data around and eventually get a desirable outcome with a framework keeping things narrowly from getting out of control when they are obvious re-factors, massive simplifications, that would get just as much expressiveness with less code. You will have bossess who only understand technical discussion in the form of "what framework or library should we use for this new epic?", like, bro, let's model the problem and see what we need from there maybe before we start thinking about writing code? You can almost just ask a frontend dev, "How would you refactor this or that code? to find out if they're just a baggage handler trying to minimize any concerted thought on their part. It's like frontend is corporate's proving ground on how to flatten all thinking and passion out of software engineering
Sorry, end of rant. I came to frontend from academia naively thinking people wanted to think about problems and put effort into solutions