Am I reading this wrong? He complaints that Valve deleted items and posts, despite them promising that it would not happen... only for the quote he provided to state that "they might not delete everything".

"Might" is not "will".

Are you referring to the usage of "some"?

> Will all of my information be deleted? Your personal information is removed, but some content you’ve posted in community areas is not. This includes things like discussion posts, or content that you posted in Steam community hubs, as well as comments you made on other Steam account’s profiles.

I read that as describing the types of content which would be deleted: "some" content but not all, and that the types of content that would not be deleted include the following: ...

I can also see how one might read that as saying that some of the following content may or may not be removed.

Yeah, I think the author was referring to community posts. The linked FAQ article says this:

"Your personal information is removed, but some content you’ve posted in community areas is not. This includes things like discussion posts, or content that you posted in Steam community hubs, as well as comments you made on other Steam account’s profiles."