A few sections later, the author makes clear that it can be taken as an execturoy interest.
Why don't they just say it in parentheses or a side note when talking about the remainders, that B would get something - it's just not called a remainder. Stupid.
We were right in the end, though.
The earlier chapters of the book are probably all about pre-1535 (or whatever year that was that they revamped everything) and then after that, the executory interest part works as you surmised.