I seem to recall having the opposite experience - vanilla username/password was the only option a few years ago, then later at some point id.me showed up as an option. If id.me ends up being required, that will be pretty gross...
It was certainly the only option when I signed up otherwise I would have been fine with a u/p on their site. Perhaps I was part of some A/B testing?
I only signed up because the IRS said they were closing the BofA/Wells PO boxes. Perhaps they were bluffing.