Legally speaking it's not stealing. But this is an ethics debate, which is probably why you inserted the word "effectively" before "stealing". That's perfectly acceptable rhetoric :)
However, I would rather characterise it as misleading Google into sending data they didn't really mean to send given the exact ways in which the data is used (i.e. watched without ads and stored locally).
So this is "effectively" giving Google some of its own medicine. Misleading people into sending them data they might not have sent given full knowledge of the facts is at the core of personalised advertising.
So is this the ad industry stealing people's data, bandwidth, CPU time and battery capacity? No, not legally speaking. Is it ethically questionable? Definitely.
So is it OK to turn the tables on them? Stealing from thiefs if you like the stealing metaphor. I think it depends, but I see it mostly as a pragmatic issue, not so much an ethical one.
I don't personally use ad blockers, because paying for everything directly is an even greater threat to privacy than advertising. It doesn't even stop tracking - on the contrary. I don't currently see a viable alternative to some form of advertising (not necessarily personalised).
But because I'm accepting ads, I'm getting to feel the full brunt of the industry's deceptive efforts to prevent me from exercising my legal rights. They are using every imaginable technical and legal trick in the book to avoid compliance with lawmakers' stated intentions.
Given this context and considering Google's oligopoly sized margins, I feel that it is disproportionate to use "stealing" as a metaphor for the comparably tiny ethics issues related to ad blocking or youtube downloaders.
We need to nudge or coerce advertising back to using context and content for ad targeting instead of running deceptive spy organisations. That would also result in more money ending up in the pockets of content producers instead of middlemen.