According to the Spanish newspapers Errijon’s difference with Podemos and Iglesias is that he is more favourable to the governing so-called “socialist” party the PSOE (which translates as Spanish Socialist Workers Party!) and wants to see a “progressive alliance” government.
Sanchez the outgoing prime minister has described Errijon as a “possiblist” (presumably as opposed to Iglesias who had raised impossible demands as a condition for supporting a PSOE government and which has led to a general election in November).
So it is “possible” that Errijon sees himself as a future minister. In other words just another reformist and/or careerist.
Perhaps Robbo in Spain can cast more light on this.