It’s been a few years since I dug through trademark law trying to find an answer to this question, but from my understanding, as long as the trademark isn’t abandoned, doesn’t become genericized, and is renewed, it doesn’t have to be strictly enforced through litigation.
You only really need to enforce your trademark when there’s a chance of it causing confusion about whether goods produced by some other party are actually produced by the trademark holder (which is the scenario your quote is talking about). Take “Apple,” for example. I can’t sell any software or electronics with the name “Apple” on it without infringing on Apple, Inc.'s trademark, but I can sell “Farmer Tim’s Golden Delicious Apples” without issue. If Apple tried to enforce their trademark on a box of apples, they wouldn’t be successful. If they tried to enforce their trademark on Tim Apple’s iJuicer Pro, they probably would succeed.
Anyway, I think a lot of the confusion about this comes from trademark law being oversimplified into the phrase “use it or lose it.” That’s strictly true when it comes to actually using the trademark, but it’s not actually a requirement to liberally enforce it.
That might be true regardless of copyright law :)
A sad truth. You don’t need to win when you can bury your opposition in legal costs (or threats of).