Agreed - not least recruiting Matz which I thought was a huge coup

At the time we would joke a bit that acquisitions would get some new very high priced domain. We even looked at getting the .app TLD for all Heroku apps to get their own .app domain. We ended up with our acquisition "gift" as Matz, which was great to see it support Ruby and the community given how Heroku couldn't have become Heroku without the Ruby and Rails community.

When you hire a celebrity programmer like that, what exactly do they end up doing?

Do they actually have responsibilities in their role or is it more just to slap the branding on to them while they continue to do their own thing, whatever it may be? (in Matz case I would think he has more than enough full time work on Ruby itself)

Heroku/Salesforce essentially just picked up the salaries for Matz and part of his team to work on Ruby, with no other commitments as far as I'm aware.