Without evidence that using an OS that isn't approved by Google or Apple actually correlates with a higher rate of account compromise or fraud, I do blame them for trying to force it on users.
I've done a couple casual web searches for such evidence and come up short. If somebody has good data on this, I don't think they're publishing it.
I'm pretty sure it's actually anti correlated: not because degoogled Androids are safer by themselves, but because the average user is way savier than the average Joe (which have an out of date Android full of unpatched security vulnerabilities, in the first place).
That's my first instinct too. I've seen a claim that devices with root, which is distinct from degoogled, both of which are distinct from using a third-party Android distribution, have an increased probability of malware. I suspect that statistic includes things like secondary devices people use for cheating in pay-to-win games and the like.
My theory is that they're trying to counter simulated phones. In the old days fraudsters would use their stolen credentials on a windows RDP rented from someone who sells fraud infra with clean IPs. I wouldn't be surprised if fraudsters have tried to develop an android based VM meant to look like a legitimate customer endpoint.
Though the most effective way to cut out that kind of fraud is to stop using SMS for authentication, which banks don't seem to be interested in doing.