It's difficult to believe this is a good faith objection.
The simple fact of the matter is that people's decisions about whether to use mail-in voting were strongly influenced by their perception of risk of going to what are sometimes crowded voting locations or to wait in long lines in them, and potentially be exposed to persons carrying the coronavirus.
Now, one candidate's supporters included a sizeable contingent who, to varying degrees, believes the coronavirus is a hoax, and their candidate was bellowing loud and long for months about mail-in voting being insecure and rife with fraud. This candidate repeatedly and publicly ridiculed people for taking precautions such as wearing masks as being "politically correct". Additionally, that candidate is more popular in rural areas where voting locations tend to be much less crowded.
The other candidate appeared to take the risk of coronavirus much more seriously, and their campaign actively encouraged people to use mail-in voting. Additionally, that candidate is more popular in urban areas where voting locations are habitually sparse and overcrowded.
It seems pretty obvious that, on the subject of coronavirus, the two candidates were actively appealing to different sets of voters, defined by those voters' perceptions of risk from the coronavirus, and that difference of perception certainly informed those voters' choices in the method they chose to vote.
Given that stark factual backdrop in the candidate's expressed attitudes toward the risk of the virus and the security of mail in voting, I can't see why you find it difficult to believe that the mail-in vote skewed more toward one candidate than the other.
