Multi-Subnet AlwaysOn: Is Routing and Remote Access always required?
Do I really have to treat this as an "AlwaysOn AG Multi subnet" even when all required routing is already implemented at networking device level and computers can all reach each other? 


Yes. SQL Server isn't doing any "routing" at the network level, it's not a layer-3 device... However, if you don't treat it as a "multi-subnet cluster" then you can get yourself into some time-out causing situations. While the situations are easily rectifiable, why not save the trouble and set everything up the first time, then not have to go back and change things?


  I've read that Routing and Remote Access is supposed to be deployed and configured but I do not get why this would be necessary since the routing is already implemented at networking device level.


I don't know why someone is telling you to setup RRAS... unless it's for some local repro environment, such as the one on my laptop that doesn't have any routing devices as it's all internal virtual machines. This doesn't make any sense unless you're using RRAS as the routing device (software router).

Either way, this is all network design and has nothing specifically to do with SQL Server.