Both US Treasury and fixed deposits are debt:
US Treasuries is debt of the US gov, Fixed deposits is debt of banks.
None of them is cash.
There is no cash backing FDUSD.
I used to think that what we could call cash is that it is short-term and highly liquid, and therefore commercial papers and treasury bills are considered cash.
I knew HB

and of course you are right, according to accounting books rules, short-term + liquid = "cash"
On the other hand:
cash doesn't bear a yield because cash is risk-free.
So where there is yield, that's not cash.
Debt bears yield because of the counterparty risk.
Debt is different from cash.
If you lend 100 yuan to Peter with the agreement that he pays you 5%/year interest and you can ask them back anytime, are those 100 yuan cash to you?
I don't think so

It's a definition issue, we are both right.
My point is this: many people believe that these $-backed stablecoins are: 1 coin --> 1$ in the bank
So when you say FD$ is backed by cash, many will understand: buy 1 FDUSD = buy 1$
Imo people should know that when they buy FDUSD they are buying US federal debt + US banks debt