But you still get faster results if you also build vocab in other ways by exposure.
My wife is considered a top experts in the field of English teaching. By the end of primary school (age 12), her students are, on average 2.4 years ahead of where they are expected to be in English skills. Her ESL students make progress at just under twice the average rate for ESL students. She has been reading the comments in this thread and almost tearing her hair out in frustration at the misunderstandings she's seeing here.
Having the English speaking kids accompanying these classes in the belief that this will be good for the non-English speaking kids is yet another example of simply using the English speaking kids as tools to teach the non-English speakers. The reason to take kids out of the classroom on excursions or incursions should be because it has an educational benefit for them not just to benefit other children. A lesson designed to build vocab in non English speaking children - and that is what this was and what it was designed to be - has no value for English speaking children.