"Bottom ranked" is the important bit here - In case multiple people without SB scores from last season win a Challenger they'd be ordered in accordance with their scores, however, all of them would be below the person with the 75th highest score from last season.
However, you're right, that is not new and does make me wonder about some assignments in the past. Rinka last season, I get, of the top 75, at least like 25 were not GP-eligible for the GP because they were either doing JGP or Russian/Belarussian, and there were 11 withdrawals among the women, but others?

On the other hand, it might explain why Kaetlyn Osmond didn't get a second GP in 2012, despite winning a Challenger as well as her only GP assignment