Too bad I can't tag this a lie :>3

This guideline relies on faulty reasoning: that since non-native speakers have less competence than native speakers, they are to be less trusted regarding any sentence; while in fact that means that the range of sentences regarding which they could be trusted is quite narrower, but within that range one can be just as competent.
I hardly need to be a native to adopt a sentence like #66680...

The guideline is based on the fact that many people are over-confident in their foreign language abilities.

That being the case, the trustworthiness may not be all that different for a sentence that isn't owned or a sentence that is owned by a non-native speaker.

If non-native speakers leave sentences unadopted and allow native speakers to adopt them, it helps show that the sentence is much more likely to be correct and sound totally natural.

If native speakers made an effort to review and adopt any good unadopted sentences, then this reasoning would be perfect. As it is though, a perfectly coherent sentence can stay unowned for years. I see it, I know it, I adopt it.
If people choose to generalize their distrust of overconfident people to all sentences owned by non-natives, it's not my fault.
I put NNC over any foreign sentence I contribute that I'm not certain about - which isn't, however, the case for the simplest sentences like the one mentioned above and usually isn't the case for the sentences I copy from a native-speaker source either.

I'm not going to argue over this.

I'd suggest following the guidelines even if you don't agree with them.
If people choose not to follow guidelines, then we'll have a less effective project.
If you think a guideline should be changed, then ask Trang to change it, but in the meantime, follow it.