iirc when I attended there were at least 2 separate communications after acceptance that stated missing the orientation meant you'd have to defer and do a different batch instead.
They make extremely rare exceptions for delayed flights, family emergencies, etc but it means they basically have to hand-hold you through a 1:1 orientation to get you caught up, which would be unsustainable if more people missed the first day. A big part of day #1 (and the first orientation in particular) is learning the social rules and seeing example of them in use, and they have a hardline policy against allowing people into the chat system, physical space, and forums until they've learned the social rules thoroughly.
If I had to guess I'd say this policy is mostly for the protection of other attendees and discourse in the internal communication streams. They don't want people going all loose-cannon on the internal Zulip until they're made aware of the rules and their responsibilities to abide by them.
They make extremely rare exceptions for delayed flights, family emergencies, etc but it means they basically have to hand-hold you through a 1:1 orientation to get you caught up, which would be unsustainable if more people missed the first day. A big part of day #1 (and the first orientation in particular) is learning the social rules and seeing example of them in use, and they have a hardline policy against allowing people into the chat system, physical space, and forums until they've learned the social rules thoroughly.
If I had to guess I'd say this policy is mostly for the protection of other attendees and discourse in the internal communication streams. They don't want people going all loose-cannon on the internal Zulip until they're made aware of the rules and their responsibilities to abide by them.