Petty and petite bourgeoisie are the same thing. In the Marxist conception, worker cooperatives are petty bourgeois. This is because cooperatives are necessarily creating class distinctions between those inside the cooperative and outside. It isn’t ownership by the whole working class, but instead a private, shared ownership. It’s for the same reason that multiple capitalists can own the same business, that multiple petty bourgeois worker-owners can own the same cooperative.

Are coops as bad as traditional firms? No. Is it socialism? Also no.