The goal is not to allow mountain bikes in every wilderness area. The goal is to allow the managers of each wilderness area to determine whether or not the area should allow mountain bikes and which specific areas should be open to bikes. There are areas where mountain bikers have built and maintained trails for decades that are turned into wilderness. Mountain bikers are then put in a position where they have to advocate against wilderness preservation which is totally at odds with how the majority of mountain bikers feel about environmental conservation.
I am a member of the wilderness society and also a mountain biker. I think that it makes sense to allow the people who manage the individual wilderness areas to decide which trails can remain open to bike use. There are benefits that bikes bring as well, particularly the amount of work that volunteer mountain bike groups put into trail development and maintenance. Where I live, the trail maintenance (fixing drainage issues, clearing debris after major storms and wild fires, etc) is done mostly by mountain bikers, not hikers or horseback riders.
Stating that the bill could pave the way for the public land seizure agenda is just fear mongering though. So we can’t change any laws because we’re afraid of what the sneaky republicans might put in them?