The reason why he did this, wasn't because of unironic support. It was due to the Alt Right and its subsequent strands growing disillusioned with Trump, he wasn't the leader they wished him to be. So voting for Biden or supporting Biden, was an ironic way of criticizing Trump.
The important thing to note is, the "dissident right" and the far right is not pro Trump, they consider him another generic republican politician who was subordinate to Jews/Israel and pandered to blacks. In essence, Trump wasn't a White Nationalist or genuine reactionary.
Interesting fact, the guy that shot up that Synagogue in Pittsburgh, posted a meme where Trump was shown taking orders from a Jewish man. I think that crudely sums ups the far right's attitude towards Trump, he's a Jewish puppet who didn't do anything for White people and in fact actively deceives them on their(the Jews) behalf.
Its important to note these people consider the whole USG and society debased and illegitimate, under foreign(read Jewish) rule, and firmly believe it needs to be torn down. So a contempt for MAGA politics and Trump is par for the course and is not at all unsurprising.
Post-election update:
So strange watching people in Spencer's sphere morph into what can best be described as "racist Spacebattlers" politically, especially with this most establishment of takes.
This reasoning isn't in alignment with the MSM per se. Its more, "believing the election is fraudulent will be a stumbling block to conservatives and MAGA folks, so long as they believe that they will never embrace our politics".