[ghaff]

>For all intents and purposes jeans and sneakers already is the lowest reasonably acceptable way to dress for an interview, right?

I would think so.

As for jacket/tie (or even suit) I take it as signaling that this is a professional interaction that they take seriously and have gone to the trouble of dressing up for. I know I'm not part of SV culture but I can't imagine dinging someone in an interview for dressing up, especially when that would still expected in quite a few places.

I also know a number of people who dress up for speaking engagements and it's just sort of part of their style.

FWIW, last time I interviewed (and took the offer), I wore a jacket and tie to my interviews even though I knew that would be a step up from the business casual or business casual with jeans that the people I'd be talking with would be wearing. I didn't need to and it didn't matter one way or the other but it made sense to me.