I think the idea is that this is for "left of the left" rather than mainstream left parties, which would still include Labour notwithstanding their current leadership.
Mostly, I agree with AndrewCA
Swiss Party of Labour - I know nothing about Swiss politics though I'd be interested in learning more; any good resource for beginners?
Key things to remember is that the Swiss far left is basically a phenomena that only exists in French Switzerland. The PdT are a party with a proud history, who trace their origins back to a split from the Socialists when they refused to join the third international back in the 1920s. They used to be the major force on the left in Geneva, but withered away, in part due to factionalism and in part as the Geneva Socialist party moved on to their turf (and general "nationalisation" of Swiss politics).
Nowadays, the PdT are basically a regional interest party for people living in the watchmaking towns in the Jura mountains region of the Canton of Neuchâtel.
Along with their new lefty comrades/worst enemies of "SolidaritéS", they are also basically useful idiots for the right, and have helped mortally wound the Swiss social security system because the proposed reforms didn't want to abolish the private sector or something.