Nope, I was just wrong. What I assumed to be a shim without looking is actually the seat tube and the mitred part is coming apart a bit.

I expanded it a little with a screwdriver, but a 26.2 seat post enters the enlarged section, but won't go deeper than 2-3cm. So, indeed, it seems it's a 26.0mm seatpost... No idea how that works tubing wise.