They absolutely are warping market forces - that’s the whole point.
We can argue about whether or not this is good (by the sounds of it, probably?) but the entire purpose of them agreeing to restrict their output is to influence the cost of their inputs/outputs.
In the absence of market power, individual firms can’t do that!
I wouldn't describe it as market forces when the organisation they're opposing is a platform with monopoly power. Etsy has fiat power over anybody on their platform.
If we're talking about the generic concept of a cartel that encompasses basically all special interest groups, then yes they're a cartel - but they're not a business cartel in the same way that Phoebus group were. The power imbalance puts them in a position more comparable to a labour union.
We can argue about whether or not this is good (by the sounds of it, probably?) but the entire purpose of them agreeing to restrict their output is to influence the cost of their inputs/outputs.
In the absence of market power, individual firms can’t do that!