Business Insider is reporting that Biden's campaign staff have been in the records as recently as March 2020:
Andrea Boyle Tippett, a spokeswoman for the University of Delaware, confirmed to Insider that individuals from the campaign have accessed the collection since Biden announced his presidential campaign in the spring of 2019. She added that the University of Delaware's library closed in mid-March due to the coronavirus, and that no one from the Biden campaign has gone to the library since its closure.
Without seeing the University's chain of custody for the docs, I don't think we can say with any confidence that absence of evidence is evidence of absence in this case.
Honestly, given that low-ranking enlisted soldiers and civilian contractors are able to circumvent the security surrounding military and intelligence service data, I'm not sure why we'd even consider the possibility that Biden's records at UD are tamper-proof.