We're dealing with people who interpret the words "price stability" to mean "prices should increase exponentially, we'd consider it a serious problem if they stayed level". It seems a bit of a stretch to go from that to believing that their mandate constrains them somehow. Their mandate can be interpreted to mean whatever they want it to mean. They're a political beast, they're going to do whatever they can get away with politically while making life comfortable for the banks.

And I'd imagine the stock market is still fairly confident that rates are going back down. If we look at a chart of fed interest rates [0] the statistical evidence suggests we're going to see low rates in the near future. It'd be nice to buck the trend and have the US stay focused on prosperity but there isn't much evidence of it yet. The basic plan of high debt then inflating the debt away hasn't changed.

[0] https://fred.stlouisfed.org/series/fedfunds