Strikes me as unethical. Unless their alerts disclosed the relationship.
He talk about something that might be of interest for these people.
It doesn't sound like he was spamming or doing anything annoying.
It sounds to me like a more human way of doing advertisement. The only difference is that he is actually there to have an actual conversation about it.
That's why youtuber put #ad on their videos
I don't know what they count as advertising per se but if endorsement via private message counts then the FTC's page[0] calls this out as "position with the company":
"Connections between an endorser and the company that are unclear or unexpected to a customer also must be disclosed, whether they have to do with a financial arrangement for a favorable endorsement, a position with the company, or stock ownership."
[0]: https://www.ftc.gov/tips-advice/business-center/guidance/adv...
From the FTC
>If you endorse a product through social media, your endorsement message should make it obvious when you have a relationship (“material connection”) with the brand. A “material connection” to the brand includes a personal, family, or employment relationship or a financial relationship – such as the brand paying you or giving you free or discounted products or services.
https://www.ftc.gov/system/files/documents/plain-language/10...
Ethical advertising is honest and up-front. If you need to be clandestine about it, you're doing something unethical pretty much by definition.
Also see: https://en.m.wikipedia.org/wiki/Lie + Find in page: "Lying by omission"