As far as I know, that makes it scale like email. When you post something, your host sends it to all recipients - everyone who subscribed to your posts.
That does not sound like a lot of traffic. Even if one has 10k followers, as the post would only go once to each of their hosts. So if the 10k followers are on 100 hosts, that means 100 messages.
Also are there any other server instances other than the php server? Seems like they ought have used the matrix protcal instead of active pub. More server implementations in different langs, and it has e2e build I. Don't think actpub has that.
But if that is their plan, why would they have used ActivityPub in the first place?
So I guess they could just shut their door as soon as the traffic is too much for them to handle. And tell people to look for another host or host themselfes.