It's a gamble because, more than likely, they have never heard of this type of account and you will be relying on their ability to understand it.

You'd be better off moving the required cash into a high-yield savings account like Marcus which I used for my application with no problem. If you really want to use your SPAXX account, you might want to give some thought to doing a cover letter explaining, in a kind of indirect way, what the account is. "Please find attached statements from my money market savings account showing more than the minimum required balance." That way you're not leaving them free to think, "WTH kind of account is this? Fidelity? Nah, that's a brokerage." Good luck!