It's plain
not correct. There's no "technically" about it.
Yes, you have misunderstood what xterm+256colour is. It's not xterm-256color. Notice the plus sign. What entries with plus signs in are is actually explained in commentary in the terminfo source file itself.
The full difference between the two entries is obtainable with infocmp, but that's just details. The simple fact is that for anything that isn't XTerm the xterm entries in terminfo/termcap are wrong. The few terminals that are close enough to XTerm to be for practical purposes compatible are the ones that were actually branches of actual XTerm at one point. All of the others, especially the recent ones like Alacritty, Kitty, and the like, are markedly and intentionally different.
The drawbacks in general are various and range from the subtle (e.g. some supposedly compatible terminal emulators mix up the home/end key codes with the find/select key codes, or they'll use different codes for higher numbered function keys, or for keys with modifiers) to the blatant (e.g. there are stark differences in indexed and direct colour support amongst terminal emulators). There's plenty of just plain odd along the way. I used the wrong TERM value for something the other day, and suddenly the Z shell's line editor was putting the cursor in column 80 all of the time.
The thing to remember is that one isn't helpless with respect to getting a proper terminfo/termcap entry installed on systems that don't have it. It's 2023, and not only has terminfo had the ability for users to supply extra terminfo entries for a long while, but historical termcap has gone away, and the few remaining systems such as FreeBSD that still use termcap can also just plonk in user-supplied termcap entries. They, too, nowadays have a mechanism for this. There's really zero excuse for not just copying the proper entry over and using it if it isn't there, nowadays. Indeed, Kitty even reportedly has a helper utility for hand-holding one through the process.