A briefing arrived. You were not in their chat, their meeting, or their week. The page is asking you for something anyway. That is the job of a briefing: to make a cold reader able to act. Your job is smaller than it feels. You do not have to reconstruct their session. You have to answer the ask, or say that you cannot.
Find the ask before you find the story
Read the TL;DR and the request shape. Decision, opinion, review, FYI. The reply is the point for them; it is also the point for you. If they asked for a pick, name an option. If they asked for defects, list defects. If they asked for thoughts and you have none worth the time, say you have no useful view. That is a legitimate opinion.
Do not answer a different question because the brief was interesting. A long reply that opens six threads is how a cold reader becomes a second author. You were not hired to finish their thinking. You were asked a shape.
Correct the guess; leave the rest
If an assumption is labelled, it is an invitation. One line — "legal did not sign off; they meet on Thursday" — is a gift. If a premise is unmarked and wrong, correct it anyway, and be explicit that the rest of their plan may sit on it.
You do not owe them a red-pen of the whole brief. You did not see the session, and you should not pretend you did. If a sentence only makes sense as a continuation of a chat you never had, say that. "I cannot answer B without knowing which migration you mean" is a complete reply. It is better than a guess you will both regret.
If they called you a name you do not use, that is their private word leaking. You do not have to wear it. Reply as yourself.
Opening is not a promise
Acknowledging is not opening. A scanner may have fetched this link before you saw it. They should not treat that as a read, and you should not treat your own first open as a commitment. If you want them to know you saw it and cannot answer yet, acknowledge, or write one sentence with a date you can meet.
You may have no assistant. The page and the email are enough. You may have a different assistant than they do. Pull the package if you want your model to help you draft; you still own the send if you use RelayLink to reply. If you reply by typing on the page, those are your words.
What you do not have to do
You do not have to watch their recording. You do not have to join their Slack. You do not have to accept a contact request to read a first briefing sent as email. You do not have to install anything.
You also do not have to be kind at the expense of the ask. If the work is wrong, say so in the shape they invited. A compliment they did not request is not a review.
Then stop. A cold reader who answers the question and goes back to their week has done the whole job. The rest of their context can stay theirs.