This scenario does not require deposit receipts that are treated as money, so what am I not getting?
Because FRB is an utterly misleading term. Banks do not create money out of thin air (as you are made to believe). It is a Central Bank that actually creates new money as required (or as it sees appropriate). Private banks only act on behalf of a Central Bank, as its proxies in creating new money
through loans. To better understand this, think of the whole banking system as just one bank with many branches, which creates new money through these branches issuing loans (as fiat)...
And neither deposits (to create new money) nor deposit receipts (to serve as money) are required, since the former are secondary to money creation while the latter can be said to "exist" as newly printed fiat
Why would Bitcoin FRB need deposit receipts (or Bitcoin IOUs) that are considered money?
Because there is no Central Bank which can print real Bitcoins (unlike fiat)