But pure AI startups are a subset of them and have probably grown along the same trend.

Self-assigned labeling and marketing info seems highly doubtful, given things like the AI Winter and the spread of AI techniques outside of whatever is deemed 'AI' at that moment in time. (Consider Symbolics selling Lisp machines: a hardware platform whose selling points were GC support, large amounts of RAM, high-end color displays, and a rich interpreted software ecosystem with excellent hypertext documentation and networking support, all in a compact physical package, sold for developers and catering to areas like oil field exploration. If a Lisp machine were sold now, would we call its manufacturer an AI company? Or would we call it a Chromebook?) And now that AI is hot, every company which uses some random machine-learning technique like random forests is tempted to brand itself as an AI startup. Trends show as much what is trendy as anything.

All the big tech companies have created big machine learning teams and hired tons of researchers...Peter Norvig commented that "his company [Google] already employed 'less than 50 percent but certainly more than 5 percent' of the world’s leading experts in machine learning".

I am sure they have, but that's very different from the claim being made. If this year you hire, say, a full 50% of a field's researchers, that's still not spending more than that field's cumulative expenses for half a century; that's spending, well, half its expenses that year.