According to various news reports, Ulbricht had approximately 144,000 Bitcoin back in 2013 and were worth around $28 million but what happened to those funds is unclear.
As it stands, I do not accept the story of the 6400 BTC as there was no source mentioned in the tweet. Which of these addresses supposedly belongs to Ulbricht:
https://bitinfocharts.com/top-100-richest-bitcoin-addresses.htmlThis might be the most fun part of the story.
Forced into a long-term hold because he was in prison, and that made him quite rich

A dormant address containing 6400 #BTC (678,400,177 USD) has just been activated after 13.0 years!
https://x.com/MatriXBT/status/1881855240393400378