Wearing a suit to a tech interview in silicon valley would without a doubt send the signal that either (a) they have absolutely no clue about SV work culture, or (b) they’re a “look at me” guy who dresses odd on purpose
If they're young it can also be because that's what they've been told to do, if they're from a different culture (even an American one) it may be shockingly weird not to wear a suit to an interview, and there are even people who wear suits all the time because a well-made suit is very comfortable, with no more showing off involved than dressing up any other way. An interview is not a regular work day, best not to summarily judge people like that.
My point is that even knowing the work culture of SV does not mean that people necessarily believe it applies to interviews too, or that a suit will be a negative point, rather than good or neutral. There is a strong culture of looking smart at interviews that overrides knowledge of day-to-day attire. If you really care about people being in casual clothes, mention it in the invite, rather than looking down on them for doing what has been ingrained to be appropriate.
