The fact that Biden got the votes he did has more to do with his opponent.
Your thesis is sound. Some of what fuels vote-rigging conspiracies are the gaps between Biden's performance and the collective performance of House and Senate Democrats. A popular candidate should produce coat-tails; Biden produced none. His repeated insistence on using divisive language is not improving his popularity.*
Vote counts can't be meaningfully compared without including total voting population (at the time) and turnout percentage in the discussion.
No matter how many times events to the contrary unfold, politicians who win will always act as if it were their doing rather than their opponents'.
I speculate some Democratic "doers" and "influencers" squinted a bit and saw 2008 again: presidency, House, Senate. But in 2008 the president was young, energetic, popular, educated, bright, and a bunch of other positive things that Biden is not; the House advantage was large; the Senate advantage was (briefly) 60 seats; Pelosi was 12 years younger and undoubtedly still in her "master strategist" prime; Harry Reid was an undisputed master of Senate procedure. None of those applies today, but Democrats are acting as if they should be able to achieve the same kinds of legislative results.
*Same question as always: how much is Biden, and how much is speechwriters? But Biden has always been prickly, and good presidential speechwriters write to suit the character and manner of the president.