Strange as it might sound, an "incontestable trademark" is still vulnerable under a host of circumstances.
Becoming "incontestable" eliminates "fair use" and some other defenses, but does not protect against arguments of genericness, functionality, certain prior use circumstances, and some other things.
Whether or not these apply in this circumstance, I don't know and frankly, it seems reasonable for Busse to try to claim a hole in a particular portion of an integral guard on a fixed blade, etc. --assuming that's what has happened ...
It's been a long time since I looked at it, but IIRC, I'm not sure a random judge would see the difference between a Busse hole, Syderco Hole, or regular lanyard hole given a plain reading of the TM.
If it was my mark, I'd prefer it was a little better defined.
It's nice to say, "Hey, I have claim to this whole universe of features because of this really broad language!" but at the same time a really broad mark reads onto a lot of stuff and that dramatically expands the relevant history of prior use, among other things.
Regardless, it seems to me Busse actually does use the hole -- positioned pretty much 'just so' -- on his knives as a trademark and has done so consistently for many years. I don't think he has attempted to curtail the use of thumb-opening holes or lanyard holes, etc.
Maybe it's just me, but in this situation, I wouldn't use a hole in similar position to what Busse does, if for nothing else, out of deference --- even if he didn't have a registered TM.