Wolfgar, thanks, but I'm not so much looking for official confirmation directly from one of the game's designers (granted that'd be lovely if it happened), nor am I looking at all for another player's personal take on the rule. I asked in case other people had had the same question as I gave above because I and another player certainly found the rule just ambiguous enough to make us question it (another player was giving the "plain language" argument as I mentioned above), and if we found it that way, minority though we might be, others might also be or have been in the same boat and could tell me how they resolved the issue.
Julia, my thinking runs like this wrt the "plain language" in the game itself (bear with me as I don't know if the copy of the rules that I can get online is the same as the one in my friends' game, and I don't have access to the game itself at the moment, so I can't look at the monster and equipment chits):
Page 24 states under Physical/Magical Immunity that it's "A Weapon or Spell that adds a bonus of the resisted type provides none of its normal bonus." I can understand a creature with physical immunity not being able to be damaged with something like a pistol, and I can see a creature with magical immunity being immune to the spell "Dread Curse of Azathoth"; both circumstances are quite clear cut. However, what about a weapon that combines magic and the force that causes physical weapons ie kinetic energy to hurt others? If I attack a flying polyp with the magical sword, it's a physical attack (enhanced though it might be with magic) and therefore should have no effect; if I attack something that's immune to magical damage with the magic sword, it's a magical attack also and therefore should have no effect. See the confusion on my part? I don't remember if the copy of the game we were playing with actually stated "magical" anywhere as more than "fluff text" eg "Magic Sword" on the sword's chit (in the same manner it does in my copy of the first edition of the game when Chaosium published it; in that particular case, it's pretty unambiguous to my understanding) as you describe it as a conceit of the game that you're just supposed to follow, but I don't think it did.
As for your digression, I've been playing only first edition AD&D (and was introduced to it by the Moldvay "blue book" edition of basic) for most of my gaming life, so I don't know what's written in later editions. IIRC at the moment, the official wording I'm used to quite unambiguously points out that "X creature can only be hit by +Y magical weapons or higher", not simply that "magical weapons are required to hit X creature" (which is usually a paraphrase of the rule that usually does a good enough job [albeit see below]). I don't remember where exactly at the moment, but I have read rules (perhaps unofficial ones) wherein magical weapons could have their pluses reduced due to physical damage. If that were to happen, and it was taken as official that "only magical weapons are required to hit X monster", a player could have a +0 "magical" weapon (or even a -1 cursed magical weapon) and confidently expect to hit a Type VI demon with a good enough roll. Do you see how the wording of a particular rule can foster at least some ambiguity?
So if the majority of people just accept the rule as a conceit of the game as you, Tibs, and Wolfgar are doing, that's fine and an answer in itself. It's not exactly the answer I'm looking for (since it doesn't match my question quite right), however, so I also ask if there's official endorsement of that ruling.