Looks like the topcover stripped all the mud off the bolt carrier into the action... One of my primary guess in CP's thread... Overall, I'm not a fan of this InRange test, but YMMV... It is an interesting test, but in any real world use, unless one is in the middle of combat, the moment a weapon dropped into quicksand or similar, one would clean it asap...
Here, the issue was the bolt not going into battery... Remove the mag, grab a canteen and flush out the forward rails and chamber area. Run cleaning rod or boresnake through the bore and then get back to testing. Overall this test design seems shortsighted... Stuff like sand tests, frost test (freeze, take weapon out to warm up where moisture condensates and freeze again), etc, type of tests seem better measures of reliability beyond the standard round counts between required cleaning, etc, type of tests.
That said, I've been thinking about this and they talked about it in the video text edits too...
OOW calls the VZ58 striker a "linear hammer"...
With striker fired pistols, the striker and firing pin is one piece, and often pulling the trigger assists in cocking the striker...
With the VZ58, it's two piece. I really think "linear hammer" is a far better description for this action than "striker." While perhaps it's in the weeds, I think it's important and will be changing my references when discussions arise regarding action type -- for parts, I'll probably stick with striker when discussing the "linear hammer" but either works... Again, YMMV.