RDPlus is an alternative to Microsoft RDS.
If you are using Microsoft RDS, you must purchase and install RDS CALs. However you are not obliged to use Windows technologies for such functions.
If you choose to use RDPlus as your remote access solution you will not require Microsoft RDS or TS CALS, only RDPlus CALs/Licenses.
Essentially if you are not using Microsoft RDS, you do not need to purchase RDS CALs.
What you are accessing is the services/sessions created by the application (RDPlus). Anyone is allowed to use VNC, Apache web server, LogMeIn, Teamviewer to directly or indirectly access a Windows system and to interact with the graphical user interface without purchasing TS CALS.