You know, I discussed this with robin too, and I don't understand this at all. The voltage potential across the holders is less than 60V running in series, and while there could be concerns about arcing to -ground- underneath, it's a full 1 mm air gap from connector to heatsink, which is much too large for this to happen.
I'm dubious until I hear an explanation, although I might have just given one. Could be a UL minimum gap requirement.
Having said that, I'd certainly edit my build thread to reflect this, if I could. Which I can't. If somebody dies, you know who to blame.
i feel ya. in an extreme perspective of thinking, the potential from the holder contact to the chip contact is zero and the (acceptable, less than 60V) voltage drop occurs entirely across the chip and again zero potential between the chip and the contact on the other side
but contacts are rated for current, where voltage rating has 100% to do with the maximum voltage a piece of hardware, receptacle, or in this case holder is rated to isolate relative to a nearby ground, to prevent arcing as you suggested
so in this case where you have a potential of 200-400V between the holder and the chassis/heatsink it seems prudent to consider that. and keep in mind ideal specifically touts that theirs are 250V rated and suitable for series installs, so at least ideal is thinking in open circuit voltage to ground perspective.
in my mind the fact that they either "rerated" or typo'd the 3070 holders from 60 to 150V (and the fact that there are shitloads of low-current high voltage series setups out there with BJBs), means it is "probably" ok in a proper install in the 150-200V range but id be really leery to run it in a setup with 250-400+V open circuit voltage.
i guess the worst case scenario is a chip ground fault (on a metal base chip i.e. most mfrs. other than the ceramic crees) and suddenly youre putting 400V to ground, you'd think the driver would overcurrent fault in this case but what would happen right before that?