You’re missing the point, it’s a relative scale. You’re talking about far right relative to all the parties. I’m talking about relative to just the Democrats or just the Republicans. Think about the current political parties in the US as a pair of just barely overlapping bell curves. The left bell curve is the Democrats and the right bell curve is the Republicans. With each of those there’s a central and a far right and far left. Trump is far right for the Republicans, and so far off the chart for Democrats to not even be visible. Biden is far right for Democrats which puts him on the far left for Republicans. Bernie is far left for Democrats, which much line Trump puts him so far off the chart for Republicans that he’s not even visible.

FPTP makes the most viable candidates for either party the ones that sit at either end of those bell curves. You either need to be as far away from the other party as possible or so close that you practically qualify for the other party. Landing in the middle of either extreme results in a loss during either the primaries or the general election.