That's quite a generalization. Yes, there are fanatics out there - that's the case in any demographic. You'd be hard pressed to look at Apple's community and not see a level of fanaticism - same with the holy war that is iOS vs Android.
Speaking as someone in the software engineering industry, I can say that a growing number of professionals (and hobbyists) are choosing to use Linux as their main work OS. Not because of novelty or fanaticism, but because it saves them time and/or does things Macs and PCs don't. I have a recent Macbook Pro and a Linux/Windows dual boot workstation - of all three, I genuinely prefer Linux. If Linux didn't save me time and money, it'd be out the airlock yesterday.
Here are my observations:
The line between designer and developer is starting to fade, especially for startups where one person wears many hats.
While Windows and MacOS are good for designers that have to dip their toes into development, Linux is a common platform for developers who have to dip their toes into design.
The low-risk/high-reward nature of SaaS services is pushing hobbyists and would-be entrepreneurs closer to Linux every day.
More and more platforms that involve graphical design (game engines, web application frameworks, et al) are starting to support Linux natively.
There is an obvious and gaping hole in terms of a viable photo/vector editing toolkit for Linux, and there are people willing to purchase a product now that will solve the problem.
Eventually, someone will make a cross-platform product to fill this gap.
I'm not bashing Windows or MacOS - they have their strong-suits and a lot of people earn a good living on them. But there is a growing demographic for Linux that shouldn't be ignored or scoffed at.
At the very least, I can say that my company would purchase a number of Affinity Design licenses for Linux, and a few iPad licenses to go with it, if it were available.