'engrossed' is not the appropriate word for your sentence when you write "…are sure to be…"
I wouldn't go along with that. There's nothing wrong with "engrossed" being a complement of "sure to be". These examples are fine:
Pat is sure to be engrossed in her homework. Ed is sure to be engrossed in his hobby.
But I agree that "engrossed" does specify "in" as a prep, though you will come across "with" and "by" quite often. Compare these two examples from the Cambridge Dictionary:
She was so engrossed with the book that she forgot the cookies in the oven. They were so engrossed in/with what they were doing that they didn't hear me come in.