What he calls RDP is a machine with Windows installed and probably a resold RDP license for multiple users, all sharing the same resources.
If anyone is selling this nowadays is a complete sysadmin nab. Most likely somebody renting his home computer for some reason.
The only reasoning I find useful for what he calls "a RDP" is for example, you have a group of 4 friends sharing multiple tools like SEMRush, Ahrefs, and all 4 connect to that Windows machine each with their own user (for concurrency).
You know that any of your friends is going to do shit with the machine, only for semrush or other tools purposes. Like a home-made GB with your friends. I used to have this like 3 years ago and was pretty neat. No downtime, etc... and only a little bit more expensive that classic GB that tend to drop constantly.