You don't even know what fed credibility means. It has nothing to do with whether you agree with their decisions, like keeping interest rates low. It's about the fed being non-political and independent
You can say anything you want, but fed credibility currently exists, as measured by bond rates which are determined by the market. As credibility fades, you can literally see bond rates increase, which is what happened on "liberation day" when tariffs were announced
The fed lost credibility when it ignored obvious inflation signals that Stevie Wonder could see that keeping rates low after injecting trillions into the economy will result in massive inflation. I have an econ degree and work in a field that actually uses it, but to be honest with you, you don't need that., conventional wisdom is better.
I know what the 'fed credibility' talking point is because I've literally heard it word for word as you framed it spammed on the mainstream media. It's hardly and afterthought in seriousness of things that actually matter.