This depends on your individual history of opening Marriott cards at both American Express and Marriott.
My reading is that you cannot open Boundless while you hold Bold -and- if you close Bold you are not eligible to open Boundless until 24 months after you received the Bold bonus.
I read that as not being able to get boundless if you received signed up bonus within 24 months, not held within 24 months. Meaning if you have Bold and received SUB 25+ months ago, you only need to wait 30 days