It needs to be pretty high too because the escrow percentage basically sets the breakeven rate of attackers. Find N accounts that qualify for this with roughly equal values, submit false claims into the escrow, if more then Y% go through (where Y is the percentage required to make a claim) the attacker breaks even or better.

On the perverse side OP actually wants a bit of spam because only a fraction of the escrow is awarded to the wallet on a successful response according to the paper, the rest is going somewhere, likely going into OPs accounts if I had a guess.

There's 3 sides to the optimization of the escrow percentage required for a challenge: 1) you want it high enough to discourage loads of spurious challenges, 2) account owners want it low enough to make recovery feasible without raising tons of money to dump into escrow, and 3) OP wants plenty of money flowing through the recovery contract since they're getting a cut of failed/invalid recovery attempts.