Pretty much yes. With developer mode on you gain access to a terminal with bash and other expected utils. There is a mobile terminal application with custom keyboard which is more or less usable, you can also ssh into the phone without any problem. As for the package manager there is pkcon (probably some alternatives are also available), you can install build utils without any problem.

I've been using Jolla for some time before moving to iOS. It was a very cool experience. I really liked having a phone that I could hack and mess around with. The gesture based UI was nice, it was amazingly comfortable after getting used to it. The main drawback was the lack of apps and the reliability of the phone. The Android apps work poorly, they are sluggish and plenty of apps are not going to work due to lack of Google Play Services. Also with Android apps you get the old flawed Android permission model. The phone crashes from time to time, sometimes in the least suitable moment (stability varies between updates).