pretty sweet trail on the first vid!
Good sport applies to both faster and slower riders.
In vid 1, front guy should have let them past. Not even a race. Pure and plain trail etiquette.
As for a race situation, if the slower rider was being lapped by the faster rider, so not competing for position, then he/she should let them go past. If however, they were competing for position, faster rider shouldn't expect slower rider to 'move' out of the way, but rather wait for a safe (for both riders) opportunity to pass and then announce that they're passing. Slower rider should then hold their line and allow the overtake to occur.
I know that's not what your MTBA quote says, but it makes sense. In racing, one of the most important times is the time before the first bit of singletrack to prevent hold ups. Its part of racing tactics. MTBA isn't going to say that slower riders should hold their line and not move, as 90% of Australian MTB riding isn't racing, in which case the rule they suggested applies.
If you like you can let every rider go past in every situation, your not going to get on any ones bad side doing that... But I doubt in a race situation, many others will.