If I don’t know someone’s gender and can’t be bothered to look it up I’ll use “they”. And I know Sam’s of both genders.

There's nothing grammatically incorrect or ambiguous about the sentence. What's causing your confusion?

I no idea who Sam is, but I believe "their" is typically utilized to describe plurals while "his", "hers", "its" describe the singular.

My initial reading led me to believe that qntm was a group aliased as Sam.

Their usage of "their" aligns with how English has been used for centuries (Wikipedia says 14th century), but more importantly the author's writing was the topic of discussion not their gender.

Policing other's pronoun usage is ideological, but that's your argument, not theirs.

Obviously, that is: statistically, all involved here (the SCP author, the HN commenters, you, me) are men, so saying "he" is very likely correct. If by chance someone here is nonetheless a woman she will probably speak up.

'Their' is practical not ideological - Sam could be Samuel or Samantha, and I didn't know which. Also, bring up the use of 'their' for unknown gender with Shakespeare and Austen, who both did it.

The only one being 'ideological' here is you, getting worked up over basic grammar that's been around longer than Modern English. Go out and touch some grass.

Sam can be short for Samantha or Samuel. "Their" allows one to avoid mis-gendering someone when in ignorance of their gender.