Socialism traditionally meant workers control over production. To the degree that Mondragon is democratic it is socialist.
Socialism came to be commonly associated with "State-capitalist monopoly" (Lenin) later in the 20th century but that does not describe revolutionary Spain at all.
There companies were mostly worker owned, democratically managed and freely associated within democratic regional and industrial cartels representing consumers' and producers' interests respectively. Highly advanced democratic systems. Larger companies were expropriated compulsory. Yes. But they were transferred into the ownership of the workforce. While smaller companies generally were free to operate as "private" enterprises though at a certain size that rarely made sense because all of industry was structured to serve democratically managed industrial operations.
