How many 1 hp orcs are there in B2 KotB? I haven't gone to check, but I'm pretty sure it's more than zero (after all, over the long run of play one in eight rolls of the hit die for a 1 HD monster will give you 1 hp). And there's a strong implication that those are all adult orcs.
As written, there are some.
And it doesn't wash there either, for two mechanical reasons and one realism reason.
The first mechanical reason: 1e for some unknown reason doesn't give monsters the benefit of their physical stats. The average constitution of an Orc is fairly high, if one extrapolates from what a part-orc PC gets, but they never get any h.p. bonus from it. Giants etc. are even more egregious examples of this. Long ago fixed in my games but it took until 3e for official D&D to catch up.
The second mechanical reason: in 1e as written death comes at 0 h.p., meaning a 1 h.p. creature will die on any significant injury even if such isn't life-threatening e.g. a broken arm. Using the optional die-at-negative-10 rule sorts this out, though one has to house-rule in means for creatures without access to magical healing to recover from significant but non-life-threatening injuries.
The flavour reason: as previously noted, a typical medium-size (or worse, large-size) creature has to have a bit more going for it than just a single hit point in order to realistically survive to adulthood. We fixed this by giving all creatures what we call "body points" (some other games would call these "wound points") which are rolled once and then locked in*. The normal hit point rolls done by classed and-or levelled entities e.g. PCs, or from monsters' hit dice, go on top of these as "fatigue points" (a.k.a. "vitality points"); with the total of b.p. + f.p. being your hit points. This gives an average-size average-con creature about 2-4 hit points plus whatever it gets from its hit dice, thus the weakest of orcs will still have at minimum 3 h.p. (2 b.p. + 1 f.p.), allowing in the fiction and backstory the creature to have survived an occasional significant injury.
Your race determines your b.p. roll and your con, if anything other than pathetic, sets a minimum value. A human rolls d5 for b.p. with con 7 or better giving a minimum of 2 (i.e. if you roll 1 it becomes 2) and con 15 or better giving a minimum 3. A dwarf or part-orc rolls d6. An elf or hobbit rolls d4. Etc.
A typical giant might have 10-20 b.p. just from sheer size and bulk. A leprechaun would almost always have just 1 b.p. as they're so small and spindly; though a really tough one might have 2.
* - unless something very unusual happens that can permanently affect b.p., e.g. loss of a limb.
Lanefan