For what its worth, Kevin from NERD says they do the spider x spider crosses all the time and there's no such thing as a lethal homozygous spider mutation. He states this ~1 minute in on the Lethal Combinations Part 2 video (not sure if links are allowed).

Assuming there's no lethal combination, sometimes the simplest explanation is correct (Occam's Razor). There are several unnoticed homozygous spiders spread out amongst the sea of ball python keepers. Their offspring are quickly turned to het spiders by spider x non-spider crosses. Over the past half year, I have read several posts (not a lot but some) by people saying they have spiders or know of people whose spiders produce only spiders but they are quickly drowned out by dissenters. That may be all you get, not some grand coming out by a large breeder with 3 generations of documented records, but just regular people who hit the odds by chance.

I'm not saying I believe this but I just wanted to throw it out as something to consider along with Kevin's take on the lethal combination hypothesis. Personally, I'm leaning towards the alien abduction theory