I'm wondering if it's really all floss.

Some of those threads look a little long maybe? I break off about 8
inches at a time, I'd guess.

But mainly, how did it get there? This is the basement toilet, which
you don't use because it clogs. So that suggests you don't put floss
down it.

But we know it's not the main line, because your other toilets don't
clog. So they are not flowing past there. You must put the floss
down your upstairs toilets, and it should go right past the basement
lateral and into the municipal sewer.

What am I missing here?