1.a3 isn't a bad move. Certainly not because of some mythical "principles". It's probably suboptimal in the sense that it (relatively speaking) doesn't restrict opponent's flexibility with regards to correct plans. Likewise, you have fewer structures to choose from that challenge your opponent's setup. For example, after 1.a3 e5 you probably have to play 2.c4 to get a position where a3 is unambiguously useful.
These considerations of course get more important the stronger you (and your opponents) get. That's why grandmasters don't (usually) play any random gibberish. You win games by setting your opponent problems they fail to solve. Which generally has to do with how flexible and mistake-proof your and your opponent's position is.
But those are still metagame considerations. In a practical sense, 1.a3 is not my recommendation. It's not a mistake, but your opponents are more likely to crumble if you play 1.e4 or 1.d4 or 1.c4 or 1.Nf3.
But also to say it matters to any significant degree under master level, is disingenuous. I get that it's easy to oversimplify chess in a convincing way to amateur players. Follow these guidelines etc. But those should be helpful heuristics, not gospel.
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