You are correct, and downandout is wrong. The same fixed point calculation that he/she makes with fractional reserve banking is true of the p2p loans marketplace as well. Your original calculation (reserve requirement of 10% -> $100 of deposits becomes $90 of loans) is correct.

Edit: Downandout isn't wrong, it's the GP userbmf that is, since he/she said that market lenders aren't banks and "cannot use fractional reserve banking", which is a ill-defined statement. I erroneously assumed that the two users were the same. My apologies.