Johnson's whole schtick is "Don't Die", so "nation of immortals" is a logical consequence of that.

See, for example, his book he wrote: https://protocol.bryanjohnson.com/DONT-DIE-by-Zero

Or a NYT piece from earlier in the year which was heavy on that slogan: https://www.nytimes.com/2024/01/12/business/bryan-johnson-lo...