Yes, their desktop clients were mainly written in Python (with Qt) and shared a lot of code between platforms.

Right now I'm looking at their installation folder under Windows. It still comes with python38.dll, as well as a whole bunch of .pyd files. There is some .exe of course, but that could be a small wrapper written in a different language. Anyway, a bundle doesn't make much sense in this case.

Although I can't be bothered to look for sources, I remember reading that they used a quite heavily modified interpreter, at least for obfuscation and optimization purposes.

Some clues can be found on their blog: https://dropbox.tech/application.