That option is for when you add it to your cart. You get API access with the device because you have one, but AFAIK you can't develop plugin without the dev option? I don't know, I'm confused :(

Interesting, I have developed plugins for my TRMNL and I only paid for the device.

Or at least I've developed HTML-based views that are custom (I think that's "plugins") that are displaying on my device.