I see, so far, little reason to believe there was any large scale election fraud this year. At the same time, I cannot take seriously the claim that this year's election was the most secure in American history.
I think states did a commendable job of dealing with a rapid shift to much higher levels of mail-in voting. When Oregon inaugurated vote-by-mail in the late '90s, they did so incrementally, first expanding absentee balloting, then trying a full mail-in election for a lower turnout state and local election, by which time they had experience in handling larger volumes of mailed-in ballots by the 2000 presidential election. With states building up their capacity on the fly this year, I am surprised it went as well as it did (I suspect the extended early voting that many people took advantage of helped with that).
But to make such a big changeover and have it managed so well that election security was higher than ever? That's an additional layer of challenge, and I remain skeptical.
Note that the security not being at record levels doesn't itself indicate electoral fraud. It doesn't even mean the security was bad. It just seems an overstatement for which I haven't seen any evidence.