The opening bit “marketing term that loosely...” clearly explains that it’s pretty arbitrary when a service is called “serverless” and that decision is usually just a marketing one. Which is precisely true.
There’s no formal definition for “serverless”; it literally is just a marketing term that loosely describes shared tendency environment and The only reason Heroku isn’t “serverless” is because they don’t market themselves as that.
So it’s a pointless exercise nitpicking anyone’s definition since there isn’t an actual formal definition. Eg AWS uses the term serverless to describe other services that isn’t lambda.
The whole term is just made up marketing bullshit for shared tendency.
In the 70s we used to call it time sharing. But I doubt many people these days will remember that term.
A commonly made distinction between the two is that Heroku still exposes instances to you: You buy a number of "dynos". Whereas a "serverless" solution doesn't, e.g. lambda just spins up workers as needed and bills you for the CPU time used.
Serverless is marketing term that loosely describes a shared tenancy environment where you don’t manage the host.