Usually it takes a large amount of dissatisfied, paying customers nagging you with bug fixes and feature requests to eventually build a quality pro product.
However, by the looks of it TagSpaces seems very polished and useful right off the bat.
You expect to type PRO and have people know what your talking about? I certainly don't, and unless you are talking about the wikipedia entry https://en.wikipedia.org/wiki/PRO_%28category_theory%29, which it doesn't sound like, I don't think it's reasonable.
They mean the Pro version, as in – the paid version of the app. I think only the free features are open sourced.
It was obvious to me even before I read the article (which has a "pro features" section).