I'd argue that healing word is explicitly healing as are healing potions and some other spells that are so flavoured. In any case, in the narrative the there must be some in-fiction indication of missing HP or otherwise there is no in-character justification for use of HP replenishing things. There must also be some in-character understanding of how these HP restoring things help.
"Plot armour" by definition is not something the characters in the setting can be aware of, nor something that can be replenished via in-fiction resources. That's the crux of my objection.
The main problem in return with this is that, because hit points are somehow
associated with injury, but are
completely injury agnostic--to the point that the exact same injury to the exact same person, but only after "gaining experience" (aka levels, which if you think "plot armor" shouldn't be diegetic, I can't imagine you think "gaining extra HP in chunky integer amounts" can be diegetic either), will have varying levels of HP impact. This is strongly related to the whole "I still have 1 HP" issue, where you can be burned, struck by lightning, fall off a cliff, etc. but it's the vicious house cat that deals 1 HP damage that actually "kills" you. If HP represent injuries, they do so in a bizarrely nondescript way.
In general, the best way I've found to resolve this is to see HP as measuring either
lack of fatigue or
internal homeostasis.
The lack-of-fatigue side has two major issues. One, it's a bit hard to see why "got brutally stabbed by an orc" and "fell off a small cliff" result in pretty much exactly the same types and amounts of "fatigue" as "immense psychic damage from magical satire" or "got struck by magical lightning...twice." Two, "fatigue" isn't exactly
easy to see, and it doesn't seem to have any association with
healing anything. On the plus side, it's usually pretty easy to know when
you yourself are fatigued, and even to have a rough estimate of exactly how tired vs fresh you are, and it doesn't require fancy words or anything. It's also understandable that people would get progressively more tired over the course of a day of hard work, but feel refreshed the next day, after food and rest.
The internal-homeostasis side has one major issue: it's not easy to see from the outside
or the inside (shock can easily go unnoticed). However, it has two
major benefits. Firstly,
this is actually somewhat like real life. In real life, if you can prevent someone from going into shock due to a major injury, their odds of survival usually skyrocket. Shock can kill people who have survivable injuries because the body just can't keep
going, and (believe it or not!) you
literally can keep someone from going into shock purely by speaking to them (keeping them awake, putting them in the right emotional state, giving them a motivation to follow, etc.) Secondly,
there's only so much a person can take. This is part of why I loved 4e's Healing Surges mechanic: Surges can't keep you alive by themselves. You could have your full complement of daily surges and still die to massive damage (instant death at negative bloodied, so taking 151% of your max HP = death). Likewise, there's only so much the body can do to keep homeostasis going, and once it's tapped out...it's tapped out, there ain't no more, exactly the same as running out of healing surges and thus getting teeny-tiny heals rather than beefy ones. It makes a great deal of sense to me that someone would know both "I am currently not well, but I can get better if I rest" and "I'm doing
okay right now, but I
have no spoons, sorry guys, I can't give more today."
But if you're going to demand a
physical origin to hit points, those seem to me to be pretty much the ONLY options that don't cause
other diegetic problems as a result....and one of the two is something a lot of people claimed they hated, while the other is still preeeeeetty diegetically iffy!