Internationals (when not functionally moribund or close to it) are mostly for networking and drinking rather than great statements of ideological purity. There have been strong personal connections between parts of the Democratic Party and parts of some European social democratic parties since the start of the Cold War*; this is mostly a formalisation of a longstanding relationship, rather than the start of a new one.
Of course it's incongruous, but these things tend to be. Now for irony, note that the Democrats have (and for a while now) been a member of the 'Alliance of Democrats', which also includes the Italian PD. Who are (of course) the legal successor to the PCI.
*Perhaps inevitably, Sam Watson - the Durham Miners leader and Russophile Cold Warrior - was a key figure in this.