IMO, it was a bit more complicated than that. I'm not sure there's a way to cleanse something so small as DNA from down inside that kind of groove (designed so as to get narrower and then wider to accommodate the snap - it also is a great hiding place for skin cells).
And I don't think he felt okay using bleach on it (that would have made it obvious he had tried to rid it of DNA). But even bleach cannot destroy DNA easily. It's a fairly tightly bonded molecule.
If he had dunked the whole thing in bleach, I'm not sure even that would have worked.
Keep in mind that there was only one DNA pattern in that snap (and they found more than one set of chromosomes, IIRC).
The above article just points out that even bleach cannot destroy DNA in a way that is consistent and regular. There are a ton of other studies. Soap won't do it. Isopropyl alcohol won't do it. Not entirely, anyway. Maybe after 50 washings with bleach - but if the sheath had been repeatedly soaked in bleach, that too could be discovered (and still could if the Defense wanted consumptive testing of the sheath - which, apparently it does not want).
I'd bet a lot of donuts it's because the Defense knows a few things we don't (such as...there's a separate trail tying Kohberger to buying a Ka-Bar knife; perhaps even showing it to someone...)
IMO.