Not entirely true, straight from his website "You can collect both SSDI and $1,000 a month. Most people who are legally disabled receive both SSDI and SSI. Under the universal basic income, those who are legally disabled would have a choice between collecting SSDI and the $1,000, or collecting SSDI and SSI, whichever is more generous.
Even some people who receive more than $1,000 a month in SSI would choose to take the Freedom Dividend because it has no preconditions. Basic income removes these requirements and guarantees an income, regardless of other factors." https://www.yang2020.com/what-is-freedom-dividend-faq/