That's really hard to say at this moment, but we can speculate. In my opinion Proto-Greek was already spoken and well developed/differentiated around 4 kya, so I think the common source of Greek and Armenian (and probably other ancient Balkanic & Anatolian languages, like Phrygian and perhaps Daco-Thracian) is even earlier, possibly Late Yamnaya or - I think more likely - Catacomb, but even before the expansion of Sintashta. But I think the closer link to Indo-Iranian in relation to other European IE groups may really indicate some mutual influence between Catacomb and Poltavka/Potapovka before the subsequent changes (like the wide expansion of R1a-Z93 and CWC-like ancestry with more EEF) that made steppe people even further apart from Yamnaya and led to Srubnaya and Sintashta. Catacomb was already, IIRC, more "northern" than Yamnaya and had closer links with the forest-steppe horizon, but it probably absorbd Yamnaya and mixed with it, so the links with Indo-Iranian but also with some Northwestern IE languages could maybe be explained due to that history.