One such a device is almost a half of the total HBN PoW network hash, which is around 700-1500 MHs last days. But it is only one.
What will be if someone smart and evil enough will point out couple of them to solo mining? Yes PoS is an additional security mechanism for the network, but it is still dangerous, dont you think?
HBN retargets difficulty every block. This will at least guard against trying to overload the blockchain with PoW blocks in a row that are exclusive to one miner. The retarget and the proof of stake should allow HBN to shield itself.
But the diff will rise not only for the attacker, but for other PoW miners too...and then goes the distributed PoS, you're right.
*Just some thoughts aloud*