Philadelphia normally goes for the Democratic candidate by around 80%, and mail-in voters this year are disproportionately Democratic, so it is not unusual that some blocks of main-in ballots in Philadelphia county would be 90% Biden. Note that the total vote (mail-in + election day) in Philadelphia county for Biden is exactly 80%, so there doesn't seem to be anything anomalous here.

Some districts in Philadelphia are extremely blue. 59 districts there had 0 votes for Romney in 2012: https://www.inquirer.com/philly/news/politics/20121112_In_59...

Expected. Last time round, Clinton got 82% in Philadelphia. And mail-in voting was majority Democratic in most places this year.

You'll see similar, on the face of it absurd numbers in the other direction in very red areas for the in-person vote, where the few Democrats available mostly voted by mail, so virtually everyone voting in person voted for Trump.