I think that listing is weird, because when I think of dry base rock, I think of rock that is completely dry and devoid of life. Pre-cured seems to imply that it still has life, so their ad is contradictory. It's either dry and dead, or it's not completely dry and somewhat cured but now dying because it's drying out.
I would side with saving the PWC
water and soak in a tub like suggested. I have been told by LFSs in the past to not add more than 10% of your water volume in LR
weight at a time, so if you have a 100g tank, only add 10 lb at a time, but that's probably for fully cured LR
not base rock.
I've added 70-80 lb dry base rock to an existing tank without issue, and that was after a couple days of soaking in FW
, which did result in a huge ammonia spike from rock that was dry for 2 years and rinsed off thoroughly before soaking!!