That's one very biased way to look at it.
He made the mistake, it happens. I doubt he'll make it again. And his play overall was spot on and super aggressive. Kudos to him.
I was actually rooting for Nguyen.
At least I did until the judge called that 'going to combat' doesn't actually mean moving to the beginning of combat but instead means 'going to declare attackers'.
I think accusing the guy of rules lawyering is a little bit unfair. You need to be extra careful about making sure everything is clear when there's language barrier on the match. Specially because what Cesar wanted to do (Crewing Heart and targeting it with Weldfast's ability after leaving his mainphase) was impossible.
This is more like it. Missing the weldfast trigger on the heart is fair, Segovia missed his chance by not doing it main phase. Making him miss the weldfast trigger entirely and also the opportunity to crew? That's just a good old fashioned gotcha and that's not what a feature match should be about. I don't hold it against Nguyen, he's trying to win after all, but I do think the judges dropped the ball there.
If judges are involved, you can be damn sure they're going to use RAW. It's entirely possible that the judge was called to clarify that he missed his chance with Heart and the judge realised that Segovia also missed his chance with Weldfast. And you can't say a judge dropped the ball by actually folowing the rules of the game (sucks that "combat" being a shortcut for declaring attackers is an obscure rule, but it's on Segovia for not knowing that).
Bottom line, always try to be as clear as possible about what you're doing during a game of Magic, even if there's a language barrier. The head judge even made sure to remind everyone about that in the coverage right after the appeal.