Does a thin tandem 2x15amp breaker draw 15 or 30 amps for a panel load calculation?
No.  The breakers have no effect whatsoever on the panel load calculation.
Because load calculations do not work that way.
Load calculations are based on house square footage and major electrical appliances.  Adding a Tesla EVSE has an effect.  Adding an addition has an effect.  Fewer vs more receptacle/lighting branch circuits has no effect.
Think about it. Suppose you buy a house and you find all the bedrooms and living room and outdoor outlets sharing 1 circuit.  However, the builder actually ran 7 homeruns for all these; they just landed them all on the same breaker.   It's an effortless job to split them all out, so you just get 6 more breakers. Now each has a breaker.  Did you change the load calculation on the house or panel?  Does the house now have more square footage? Did you add any major appliances?  No, not at all. It's the same house with the same loads.  There are just more breakers behind the loads.
Or, you build a house to NEC minimums with 1 recep circuit for all 3 bathrooms.  I build the same house, but because Harper, I wire 2 dedicated 20A circuits per bathroom.  Yours all fits in a 24-space panel, I installed dual 40s.  Our square footage and major appliances are identical.  Are our load calculations different? NO.

The only thing that double-stuff breaker is relevant to is stab limits.  That's 30A worth of stab load, so across from it can't be more than a 95A breaker if it's the usual 125A stab limit.