If the actually rich people are dressing down-to-earth, why would you dress fancily to appear rich? Surely you want to also dress down-to-earth so you appear like the rich people do.
An example of this is in luxury retail stores, fancy/business clothing used to be such a strong signal of high status that retail workers there would often ignore you if you walked in wearing T-shirt & jeans because they'd think you couldn't afford it. Nowadays, that's a lot less likely, they know too many people dress like this for a retail worker to safely assume anything. There are a lot of wealthy people who aren't in the news all the time and a lot of them don't dress like a traditionally wealthy person.
One approach is to dress like other famous & successful tech founders: a nice T-shirt & hoodie. T-shirt & hoodie is also just comfortable and common, which is exactly why those famous & successful founders wear it, they're not showing off traditionally by wearing $10,000 tailored suits, they're wearing a T-shirt & hoodie that anyone could wear. Dressing up with a fancy suit might actually look odd amongst the Silicon Valley upper class.
Edit: Another interesting example I thought of when replying to another comment is in luxury retail stores, fancy/business clothing used to be such a strong signal of high status that retail workers there would often ignore you if you walked in wearing T-shirt & jeans because they'd think you couldn't afford it. Nowadays, that's a lot less likely, they know too many people dress like this for a retail worker to safely assume anything. There are a lot of wealthy people who aren't in the news all the time and a lot of them don't dress like a traditionally wealthy person.