Comment by Spivak

I mean it's not really a paradox. Stallman's vision of open source is one where software serves its users' interests and no one else. This came at a time where people were buying bits-in-a-box to install on their computers or electronic appliances like printers where the vendors leveraged their position to deprive you the ability to do as you please with your own device. Looking back these complaints seem almost inconsequential compared to what we deal with today. The degree to which your computer is no longer yours is significantly more impactful than some ink cartridges or interoperability issues.

Stallman recognized that someone else's computer was going to serve them and open source was, maybe ironically, meant to facilitate that. Open source client to a proprietary service isn't some kind of contradiction, it's respecting your freedom to control your computer same as it always has been.