You have paraphrased what it says and IMO is is always a good idea to start at the exact wording:
"An ambush cannot include a general any of whose troops are not in the ambush, ..[etc]"
so no - you are not wrong - the use of "the ambush" is singular and definite - "the ambush" is that which includes the general - nto another one.
Basically your wording is simpler and accurate....but it always pays to check the original with Phil's rules!