Load of crap. No president running for reelection wants to see a primary challenge. That applied to Trump no less than Biden. Trump was not seriously challenged and thus far there is no serious challenger to Biden.
Do you see anyone on the GOP side who is appealing to both the GOP Party voters AND the wider electorate? I don't. Christy is the only one eviscerating Trump at the moment. The rest of the field is too craven to appeal to anyone but the Trump voters.
Dems have problems too when they will not support a true primary system because everyone need to bend their knee to Biden. And the Marxist wing of the Democrat Party push back.