there are too many people going on the morning coffee run at a nearby cafe at once, and that it "disrupts the work routines" of other people.
Let's take that at face value. Could your boss be right? Absolutely.
If you go out for a coffee alone, you are silent. You are focused. You talk to nobody, go over the street, order your coffee, wait for it, then grab it and go straight back. You go back to your desk and continue working.
Now if two people go for a coffee, they talk. Not loudly though. They go over, both order, then both wait for their order. After the first gets their order, they wait for the second one's order. Then they go back chatting. This took longer than them going separately, because person #1 had to wait not only for their own coffee but also for person #2's coffee, while person number two had one more person in front of them in the queue. It was also noisier. And it increased the chances that a question in the office went unanswered because although either would have known the answer, neither was there (compared to them going one after another).
Add more people, it gets even worse. The chatting gets louder, because now there are multiple parallel conversations in the group, the combined waiting gets longer, because person #1 has to wait on person #2-#7 while #7 has 6 more people in front of him/her than usual. And the chance that one of them can help with a question is diminished greatly.
So yes, it's absolutely possible that the rise in volume, rise in time spent idle and not working and time lost by others who had to wait for you to come back to get their question answered has finally risen to a level where your boss needs to object.
To see if he has a point, just a thought experiment: If you had 1000 employees and they all went to grab a coffee at the same time and all stay until they all have a coffee, how long do you think that would take? Would any work get done that day?
So yes, it's a thing. You will have to work around that. Maybe you can send one or two people with orders from all 7. For good measure, ask your boss if he/she wants one as well. That should be enough to satisfy everybody.