Biden didn’t reassemble the Obama coalition.
He reassembled the Hillary Clinton ‘16 coalition: by being a moderate, a veteran of the Democratic Senate, and having previously spent 8 years in the White House. Only Biden was less politically toxic than HRC, and the country now had the benefit of 4 years of Trumpian disaster-making where previously they had none.
Biden’s map looked very similar to HRC’s right down to the county, just with better win margins in some of the metro counties. His path to victory wasn’t in effectuating a grand political realignment, he just turned out more Democrats and more moderates.
Given the above fundamentals, Biden’s win in a high-turnout election (high turnout on both sides, since everyone is miserable this year) was totally plausible.
But keep screaming that voter fraud if it still makes you feel better.