They missed the 3rd(!) bug in those few lines of code? The normalization was done before zeroing out the Z component, so the resulting vector isn’t actually normalized.
Not saying I never conjure the spaghetti but whoever coded that really was deep into the tail end of a caffeine, coding and sleep deprivation binge ;)
That's most indie game devs of the period, but more so at id Software which splintered the team during Quake's development due to internal squabbles, causing John Romero and others to leave id.
You can even tell that from the quality of Quake's levels which start with beautifully crafted and intricate levels, and as you approach the end, progress into "whatever, let's just ship it, this is gonna sell" kind of levels that were mostly just boring repetitive filler to pad the play time.