If you search Wikipedia for plum, it is quite clear that there are various species of plants that have been domesticated and that are considered plums.

Plums are likely to have been one of the first fruits domesticated by humans. Three of the most abundantly cultivated species are not found in the wild, only around human settlements: Prunus domestica has been traced to East European and Caucasian mountains, while Prunus salicina and Prunus simonii originated in China.

What the meme is referring to is probably Prunus domestica though, where Wikipedia has to say this about:

The species’ hybrid parentage was believed to be Prunus spinosa and P. cerasifera; however, recent cytogenetic evidence seem to implicate 2×, 4×, 6× P. cerasifera as the sole wild stock from which the cultivated 6× P. domestica could have evolved.

The other two mentioned species seem to have been species that we’ve domesticated on their own, without hybridization. At least there is no mention of it on Wikipedia.