I think I'll caveat that with "in the presence of other bilinguals". I am trilingual (formerly bilingual) and I, as well as people I share >1 languages with, freely switch when talking. It's quite liberating since you get access to idioms from multiple languages, and certain phrasings work out better. (There's also a lot more potential for great humor)
None of the adult bilinguals I know accidentally switch in the presence of non-bilinguals, however I have seen bilingual kids a few years younger than me do this (and I suspect I did it too).
In the context of kid mistakes, that quote was rather jarring to me; adult bilinguals do not switch languages the same way kids do; it's a completely different thing.