You are right that it's about marketing -- getting 7 friends together on the same day is no small feat.
But the "4 is the recommended size" is just objectively false. I hate when putatively authoritative facts are at odds with available evidence. 4-6 with advice on how to increase past 6 is not '4'. Neither is it 5. It's 4-6 or more if you want.
Look at the modules themselves and listen to what DMs are saying about the official modules:
https://www.reddit.com/r/DescentintoAvernus/comments/ejqf6t/ideal_party_size/https://www.reddit.com/r/DescentintoAvernus/comments/d6tbfz/party_balancing/"I had 6 at my table last night and they came alarmingly close to getting TPKed in the very first fight.
They only survived because I ruled that the pirates ran away when their leader was slain.
If you're gonna run a party of 4, you need a well-balanced party with the Holy Trinity otherwise I think they may struggle."
