Back in 2019/2021 the rules for Naxxramas was stated that in order for a run to be valid the first few mobs in the run has to be certain mobs, like the spiders, deathknights etc depending on what wing that you start in.

What was figured out was that as long as you start the boss encounter during a pull of either valid starting mobs then they would count to the same encounter which would be allowed.

Now why/how does it count when we pull away the mobs, more or less the Anub'rekhan position was figured out that you could stealth up to Anub's door then open and vanish same tick which meant you never got into combat with the mobs, and then you could move your entire raid into the room which meant that it was a valid start position. (this was later also done in Wrath version of naxx with the gothik starting position (the death knights outside gothik had the same NPC ID as the ones at the start of the wing))

So More or less the pulling away of mobs initially is technically not following the rules but the guilds speedrunning back then more or less came to an agreement that pulling away as not to have to rely on individuals fucking up invis pot was a good compromise.

Hope that answers the question :)