There's an important shortcoming in that reasoning: it assumes that time is fully dedicated to learning whichever language is being used. Furthermore, it doesn't consider the possibility that learning a language may benefit from learning a different one.
Based on my observations while raising my kid bilingual, that's probably not the case. When she learnt a new word in a language, she usually asked for it in the other language too so she could learn both. At 6 years old, she now has much better language skills in both languages compared to her peers. There may have been a period at about 2 or 3 years old that her skills weren't outstanding but she was still perfectly fine by her age's norm (which accounts for quite a bit of variance). My guess is that, after learning a few basics, new language is learnt sporadically with lots of "waiting" time in between, and thus there's practically no time conflict in learning two languages except maybe at the very start.