What are you talking about? Biden hasn't denied Ukraine membership in NATO (it's not up to him). To join NATO you can not be in a current conflict (see:the current war.) Biden would absolutely love to have Ukraine join.
You are entirely right, but the person you're replying to was certainly referring to how the US and Germany together prevented NATO from providing a more concrete timeline and encouraged the vague promise of Ukraine being offered an invitation to NATO when unspecified "conditions have been met".
There can't be a timeline unfortunately. Putin knows that NATO won't let Ukraine in during a conflict. All he has to do is keep any sort of conflict going and he prevents NATO membership.