> Arrow quote marker is already there, so it there'd be no reason for the [sic] in my mind.

[sic] is only ever used with marked quotes, to indicate that something the reader may see as an error is preserved exactly from the cited original, so the presence of an arrow quote marker doesn't remove a reason to use it, it is what creates the context that makes it relevant to use. It is clearly used in the standard wat.

It's true that one might see less reason to use it in a medium like HN where the source is on the same page and the usual method of quotation is cut-and-paste rather than transcription. OTOH, it's also a medium where the source can often be edited after the response is posted, which can provide an additional reason to do it.