I think trying to promote your YC startup amongst your fellow batch members is different to asking the tech community as a whole to vote for your product. Perhaps "founders told each other to upvote their products" wasn't the best way of phrasing it, but I imagine it was more like how you'd ask those closest to you to give you a bit of support.

With regards to the conflict of interest, I don't see this being too much of an issue. Whilst PH itself is part of a YC batch, the majority of the PH community won't be, and it's them who collectively vote.