To be fair, they do call out in the article that they're hosting on their own hardware. Afaik Heroku has always been hosted on AWS which is a lot less capital-intensive than buying hardware.

Also, Heroku was acquired in 2010. I know that the prevailing sentiment around here is that the acquisition was a mistake, but Heroku has been owned by Salesforce for 13 of it's 16 years of life...