Yes, the explanation about the LUN spreading is completely correct, if you use Raid-1 (I think you were meaning this, not 2…) basically each side has a copy of the LUN itself. What changes (and that's the reason I suggested that link) is that at a given time only one of the P4000 is the gateway of the LUN, the others are only syncronyzing the copy of the LUN they have.
This means, in a flat scenario with LeftHand, at some point the LUN could be exposed by a storage node in site 2 but accessed by an ESXi server in site1. This is not a real problem if the inter-link between the two sites has the same speed as he local network, like you confirmed. You will only see an increased amount of traffic in the inter-link both from ESXi accessing storage across the link , and the several storage node synchronizing each other.
Luca.