Salesforce bought Heroku in the early days of the “Salesforce Platform” product era. I wouldn’t be surprised if Heroku tech underpins a big chunk of the AppExchange developer experience.
There might be less than you would think. Salesforce's security review does not take kindly to having executable portions of your managed package live outside of their stringent security process. That isn't too say it doesn't happen. But you have to make a case for it being critical.
