This is a case where at first sight it appears that BoA is in a bad situation. On further analysis it appears this is a non-issue, for the reasons you explained. But going further still, it's obvious it IS an issue. And your flaw is assuming that when a bank says "we're holding them to maturity", they're holding them to maturity.
You see, SV did the same thing. They took some assets that they planned to hold to maturity if all goes well, but then used the same assets to tell regulators "don't worry guys, we have enough liquidity because there's these things here which we can sell if we really need to". Before SV went bankrupt a naive observer could've asked the question: if all clients decided to pull money, did they have enough liquidity without having to touch these assets, and the answer would've been No.
Whether this is a problem for BoA depends on the answer to this same question. It's far from obvious that this is a non-issue.