The rest of that page suggests they haven't decided yet. They say "Barring abuse or attack, we will not impose limits errors for any application without contacting the customer. Customers whose usage claims a disproportionate percentage of resources for their current plan level may be asked to upgrade to the plan level that matches their needs." I currently have a $200/month business subscription (for https://cloud.sagemath.com), which was required for websockets before today, and will very likely switch to a $20/month pro subscription. We typically have up to about 500 concurrent connections on our site. Incidentally, we switched to using CloudFlare about a month ago after being hit by a DDOS attack (400MB/s incoming traffic).

[Edit: actually, we're not switching since only the business plan provides "Advanced DDoS protection - layer 3 and 4", which was why we are using CloudFlare in the first place.]