If all the ballots were shuffled in the mail across the timespan in which they were counted, that would make sense.

However, they are not, so the assumptions underlying the analysis do not hold.

For one thing, many states did not begin counting these ballots until Election Day, per state law. For another, the entities performing the unofficial counts were not necessarily the entities receiving the ballots. The author clearly conflates these two points when he says that Milwaukee ballots heading to the county to be counted were received after the deadline. They were received by the precinct at the appropriate time, and then transported to the county for final tabulation - again, presumably, in compliance with state law.

(EDIT: Notably, the video clearly shows the ballots being transferred by law enforcement, so it's patently dishonest to suggest that was the time they were received through the mail.)

If a state performs constant counts and constant updates as they receive individual ballots from across the state, then you should get a more-or-less constant D:R ratio.

If you count mail ballots in batches of updates from different counties and within counties from different precincts, then you will see the D:R ratio generally shift, with precincts and counties with higher populations (ergo higher batch processing times) coming later.