For speedruns, there's a philosophical divide- some people consider exploits to be cheating, thus it falls under the "no cheating" rule.
True, there are some exploits which are truly degenerate, but they are specifically handled. Exploits in general are highly encouraged. Speedruns are listed in terms of the restrictions given: a specific difficulty, 100% vs any%, rules governing the timing, etc.
Exploits in speedrunning aren't often easy to do, and finding new applications for them are part of the challenge and can set one speedrunner ahead of another.
That seems like cheating to me. Little glitches in the game mechanics are a little bit different.
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.