"Request" is not quite the same as "get." Presumably, a request will not strain the servers, and they will still serve them out at a rate the servers can handle. But some folks will want the update right away and others will prefer to wait and make sure there are no bugs. This will be better than just sending them out by purchase-date priority.
What evidence do you have they're sending firmware updates out in order of purchase date? It certainly doesn't work that way for the S/X.
I do agree with your statement about "request" and "get". Based on no substantive information whatsoever, I think that this new feature will allow a car to check for eligible software updates on demand, but individual cars will continue to have different "newest" versions, depending on hardware configuration or Tesla's rollout strategies. In other words, I've seen a lot of people reading (IMHO) way too much into Elon's tweet.
Bruce.