To put things in a clearer way, there's now a "experimental" stage and "public" stage where only Patreons fans (a.k.a. subs) have access to the experimental stage and, if we follow the point given on the Patreon's page, the public (those who purchased on itch.io) gets access to the "experimental" stage 1 month later when it turns into "public" stage.
That's, at least, the point that seems to be explained on the Patreon's page:
[Experimental], [Rough] and [Release] builds of our original games at least a month before public releases.
then later, it's written :
Public builds don't do the current visuals and features justice but you can play them on NewGrounds and Itch.io!
If I'm wrong, then I'm sorry. That's all that I could find.