Their standard, EGLStreams, is only implemented by their proprietary drivers, so in order to test that codepath we'd have to use a famously broken and undebuggable driver, which we're not interested in - and make no mistake, we get our hands dirty in the drivers all the time. Their standard also has serious technical problems - lots of really important features simply are not possible to implement based on the proprietary driver. We can't export buffers to clients, overlay planes don't work, buffer presentation timings are impossible... Supporting this driver would be a massive overhaul and would probably make the good drivers worse.
Sway is a volunteer project, we are under no obligation to put in free work to support a GPU vendor which has no interest in playing nice. It's total bullshit that anyone would think to blame the small group of volunteers who have offered up thousands of hours of our free time to the community, for free, when the real blame lays in the hands of the multi-billion dollar company which refuses to pony up docs and cryptographically signs their firmwares. Everyone else plays ball and works with upstream on open source drivers.