This is true in all editions of D&D, and the rules explicitly say so.
...sort of.
There's one paragraph in every edition of D&D that says this. If you were to black out that paragraph, however,
everything else in the rulebooks pre-4E would imply that hit points represent raw physical toughness and high-level PCs are just made of iron.
All of the terminology supports this. Anything that gives you back hit points is described as "healing," anything that takes them away is "damage" which is usually caused by an "attack" that "hits," poison and other rider effects are triggered by loss of hit points, et cetera. Hit point recovery happens either through magical curing spells or through days or weeks of bed rest. Unconscious characters still have full hit points. And so on and so forth.
Take it from a software developer: When the manual tells the user to go one way, and the software pushes them the other way, the software almost always wins. The manual gets one crack at them, if you're lucky*. The software hits them every moment they're in front of the computer. Likewise, the "hit points are not toughness" paragraph gets read once, while the rest of the rules are nudging the players toward "hit points
are toughness" every single game.
4E's designers decided that that one "hit points are not toughness" paragraph gave them carte blanche to turn hit points into "whatever is most convenient for our tactical minigame." They might have pulled it off if they had just had the good sense to overhaul the terminology. Instead, however, they left it all like it was; anything that gives you back hit points is healing, anything that takes them away is damage caused by an attack that hits, poison triggered by loss of hit points. The result was some nasty cognitive dissonance. The mechanics were now saying one thing while the flavor was saying something else.
Hence the outcry, and the complaints about warlord healing. If warlords triggered a "heroic surge" that gave you back "vitality," I doubt anyone would be complaining. In a role-playing game, the choice of what to call stuff is
really freaking important.
[size=-2]*And if you think you can count on getting even that one crack in, better think again.[/size]