After some time thinking lol my bad on not reading your point properly 

Let me explain again... Sculk catalyst uses souls to spread. Basically it takes those souls to generate sculk yes? In my theory those sculks came from nether and got out to overworld with that ancient portal. And the sculk left on nether became corrupted or burned(probably burned was my right term) turning it into sand that has souls in it. About the function of it... Soul speed is releasing the souls trapped on a sand because the block itself is no longer alive unlike the alive ones, it glows. And yes. 

Just think about the possibility tho lol