It'll be marginable in a normal brokerage account

Again, already possible. Go to your Fidelity account and check.

Source? First search result says otherwise:

"Currently, Fidelity Crypto does not support borrowing against assets, leverage, or margin trading. You can only transfer, and use collected and settled available cash, as well as processed EFT deposits, to trade within your Fidelity Crypto account"

https://www.reddit.com/r/FidelityCrypto/comments/17lr3ro/com...