The same applies to FB and Instagram as well, fwiw. Though imo, not as much for WhatsApp, which already had 400MM users and would have organically reached 1B+ users on its own.
YouTube had 65 employees when they were acquired.
I do agree that they were not a startup. This word should only be used for companies that are starting up; getting their legal structure together, hiring, and initial R&D. Once you are offering widgets (ad space), you are no longer a startup. Profitability is immaterial to startup status.
Not every company that's successful and has millions of users needs to have a bloated org
The Android acquisition is a redherring - manufacturers started adopting Android en mass because it was royalty free(ish) and they had to compete against Apple's new app store mostly with feature phone OSes entirely unfit for the job.
Neither Youtube nor Google's selfless "donation" to the consumer electronics industry would have been possible without the ad business. IANAL but that looks like textbook predatory pricing (and in Android's case, can't be defended by pointing at Apple, since they don't participate in the smartphone OS market).