I remember seeing a speedrun of Half-Life where the player went through a wall (that you weren't supposed to go through), kept going (past all of the weird clipping visual nonesense), and finally came out in a different level (which IIRC wasn't even populated with enemies), and kept going through the game.
That seems like cheating to me. Little glitches in the game mechanics are a little bit different.
The problem is that there's no bright line to be drawn between bunny hopping and glitching through a wall. It's a continuous spectrum of larger and larger glitches.
Just out of curiosity, I assume you'd also consider abusing the "minus world" glitch in Super Mario Bros to be cheating, assuming it happened to let you finish the game faster? That glitch is nearly identical to the Half-Life case you described.
That seems like cheating to me. Little glitches in the game mechanics are a little bit different.