Support added the pack to the system, they wouldn't add it if it wouldn't have Steam games as well :)
link

Pay whatever you want, and get four excellent games that will work on your Android mobile devices, as well as any Windows, Mac, and Linux computers you have: Anomaly: Warzone Earth, Osmos, and EDGE. Customers who pay more than the average will also receive World of Goo, an indie classic and an Android delight!
As per Humble tradition, these games are DRM-free and redeemable on Steam, and you have the option of designating part of your purchase to the Electronic Frontier Foundation and Child’s Play Charity. (You can also leave a tip for us, your humble game promotion organizers.)

Support adds games / packs to the list, therefore they contain Steam keys or they wouldn't do it. Link and quote in my comment above this.