It's not a binary toggle - firefox is selling you as a source of revenue for themselves. They're just not making it as extreme as it is possible to be - in the hopes that you don't switch away.
You can compare same situation with safari in iOS. Except google pays a lot more, since you cannot switch away in iOS as easily, and culturally there's more reluctance compared to firefox users. This makes google pay more for iOS traffic, as those users are worth more.
The sense in which you are the product on Firefox is that they want to maintain a large enough user base that search licensing is valuable enough to sell to Google.
Because I can change that default and still use the thing. That's how it's very different.
Typically when people say that when something is free you are the product. They mean that it's free because your data is being sold, implying that without telemetry it wouldn't be free. That's not the case here as far as I know
Google is paying Mozilla to be the default search engine. Google is only paying Mozilla because Firefox has users, regardless if they use the default search engine or not. So, indirectly everyone is the 'product'.
I'm sure if 95% of people did swap to ddg, then google may change their mind.
Also I believe there is the possibility Google also pays Mozilla to offer competition so Chrome isn't considered a monopoly (but maybe Edge has changed that to some extent?)
To give you an example. Try to use Google Search without sending your data to Google. You cannot use the product without it, you cannot opt out. Firefox, you can use just fine with Google not being your search engine.