Summary: The Recurse Center is a six week retreat by programmers. The first day of RC is mandatory. That's the day they go over how the community works, and their community guidelines which they consider to be one of the most important parts of RC. Since OP missed the first day, RC told them they would have to defer to the next batch.

IMO, it does seem like the consequences of missing the first day are a bit harsh. To be fair to RC, they do mention this is the only person to miss the first day since they've been doing RC remotely. This person is the only person out of hundreds to not know they were supposed to be there the first day.

When I went through the Recurse Center several years ago, I recall them mandating you are there for the first day. My high school graduation was the same week as the first day of Recurse Center. I wound up flying from Chicago to New York over the weekend to attend the first day of RC on Monday. Flying back to Chicago on Wednesday to attend high school graduation on Thursday. Then flying back to New York over the weekend to finish RC.

For more context, the RC team is understaffed. It's ran full time by 7 people. Notably the staff are doing this largely out of good will. RC makes enough money to sustain the community, but does not make a lot. I know they are having a hard time during COVID. I can understand why they wouldn't want to have one of their staff go through the orientation again as it would create a large burden on them.

tl;dr The penalty RC gave is harsh although they likely did communicate the fact the first day is mandatory properly and performing orientation a second time would create a burden on them.