Pease porridge hot!
Pease porridge cold!
Pease porridge in the pot, nine days old!
You can still get plenty of oatmeal in Europe just as you have been able to for centuries, so I don’t see how come your friend said it was not available historically or that even now there is need for a “modern substitute”.
Perhaps the modern gourmet wants to mix it up; why eat oat porridge every single day when you can have rice, wheat, maize, poi, etc.
In the Commonwealth English that I’m familiar with, oats or oatmeal is the dry stuff that’s stored in your cupboard. Once you cook it into something you can eat, it’s porridge (or if you stick a bunch of stuff in it and eat it raw, then I guess it’s muesli)
A porridge made with oatmeal (i.e., the bran, rather than whole flakes) is going to be pretty unsatisfying. The kind of porridge you’d get in Scotland would be cooked until the flakes release their starch into the liquid: with just oatmeal, it’d be rather gritty in texture.
In St Martin we sometimes have breakfast at the Gingerbread Cafe. On their breakfast menu they have Oat Meal (two words) for “health conscious” diners. We’ve never ordered it (nor seen anyone else order it). The idea of being “health conscious” while on vacation just doesn’t click for me. I typically go for the Norwegian Eggs Benedict (Eggs Benedict with smoked salmon) and a Bloody Mary.