Because if he doesn't disclose it and we find out, it looks terrible and like he's trying to hide something.

If I know a competitor wrote it, I don't read it as negative. To me, it's a welcome courtesy, but as you say, I couldn't assume it if absent; so I'd probably paint a negative picture of S3 with the same albeit unprefaced sentence.

(Plus, I'm always pleased to see someone not call it a 'disclaimer'!)

Disclosure of Interest should be a common practice across all Industry and web discussions. It provides a lot of additional perspective, most of the places do not adhere to this.

Fortunately HN has kept the culture of disclosure intact.

Google cloud is a significant direct competitor to AWS. Anything criticizing AWS reads that way to some extent.

He mentions Google frequently in his posts for context. It's very useful, and there was nothing disparaging that he said.