As a german I can read that tooltip just fine. That immunity is obviously for non-magical weapons only.
Opponents that can only be hurt by magical weapons, this concept is so old, its for example mentioned in Lord of the Rings. So it predates D&D.
In BG1 you had this, too. And in the BG1 addon you had Werewolves(!) that required silver(?) weapons. Or was it cold iron ? Some special material, anyway. Made fights against them a nuisance unless you knew what to do.
And in BG2 you had a lot of opponents that even needed +2, +3 and in some cases even +4 weapons; the later was IIRC only Demiliches ? There was also a spell called "Absolute Immunity" that required +5 weapons to break. I think in one case you even needed +6 weapons - but I cant recall right now when that was the case. There was also "Protection from magical weapons" which however could easily be destroyed with a simple Dispel Magic.
And you would only get four +6 weapons total in BG2 - a greatsword only for paladins, a halberd, a spear and a quarterstaff. And that exclusively happened in the addon ToB. In my games the greatsword always went to Keldorn (except if I was playing Paladin myself), the Halberd to Minsc (or whatever other character I took with me), the spear obviously went to Jaheira (dont think I ever not had Jaheira with me, I never played Druid), and the Quarterstaff could be wielded by anyone anyway.
This is by the way a really good example of what a complete PITA AD&D was when it came to weapons. You had to plan ahead many levels before what weapons you would use in the end. You had nobody with the right weapon skil, well you would suck using this weapon.
And there was no Withers for respecs either.
Fortunately there was a spell to get +6 weapons ("Melf's Magic Meteors") quite early and quite easily.
So in this regard BG3 is actually very, very simple by comparison.
And its sort of a good thing because otherwise Open Hand Monk would be extremely hampered against certain opponents.
