They do say they're GPL, and let you download the source, but they also say that 1) building from source is hard 2) they won't keep build instructions up to date 3) they won't answer questions about the build process. https://ardour.org/building_linux.html With that attitude I'm not sure why not just buy commercial.

Wow! You need to browse through 4 pages trying to download the program before they ask you for money! I think pay-for-binaries can be a legit monetization strategy for open source, but this is really sneaky.

In a way, you are buying commercial. The Ardour people are selling downloads of their binary builds for $1 per download (or more, if you choose), along with some access to support I think.

It's also possible to, for example, 'apt-get install ardour' and get a similar version in Debian Buster. I presume there are lots of other operating systems that this is built for, so Ardour does make a significant contribution to free software.