The original job postings[1][2] were even more egregious. They originally said, "many of us routinely work 70-90 hours/week". After a lot of criticism on a Facebook post by Andrew Ng[3] (where you can still find residual evidence of the "work 70-90 hours/week" claim), they modified it to, "many of us routinely work and study 70+ hours a week". So to be precise, they were initially offering you a 43% to 56% pay cut. G…
That said, I think calculating the percent pay cut feels like an awkward way to frame the issue. Very few people working 70 hours at a salaried job could get paid for 70, even contractors will normally get scaled back to 40 if they ask for 70. If you worked only 40, very few people will get another 30 hours per week job to make that extra money. (And contracts for lots of salaried employees would prevent that scenario.)
The value of hours worked above 40 is your life, not a percent pay cut. It would take a jump from a 6 figure salary to a 7 figure salary, or more, to make extended 70 hours weeks worthwhile, so I think talking about a 50% pay cut is drastically undervaluing that extra effort & time.