Having spoken with Mark privately as we run similar forums (both Football, not Soccer, Football!), he is speaking about guests viewing the forum not members.
On his setup, only guests are shown ads, his concern is that if guests are not shown up to date posts say during the game they will choose to follow the game elsewhere, say X for example.
This would obviously have a negative impact on his ad revenue which pays the bills.
I do share his concern somewhat and would be nice to reduce the time, I personally ran with 5 minutes when using Cloudflare on a Self Host setup prior to moving to IPS Cloud.
For me this was a nice balance between not annoying guests too much whilst encouraging them to also signup and login.
I guess the feedback here would be, it would be nice to control the cache time per site if possible rather than a blanket uncontrollable 15 minutes which maybe fine for say a gaming forum, just not for one that has live events.