For those who don't read French: that construction could roughly be translated as "the following game was supposedly won [or: 'is said to have been won', etc.] by Einsten against the great physicist Robert Oppenheimer".

The conditional tense here in French is used to report that something has been asserted by someone, without taking a position one way or the other on whether the assertion is true. I don't think there is an exact grammatical equivalent in English.

Something like "which would have been won [if the reports are to believd]" perhaps.

I think in English “would have been won” implies that it was not in fact won.