One can assume that, but that's not what it says. The statement that the editor is free during beta could mean "we haven't figured out whether to sell it after the beta, sell services around it, or develop integration with our other services and keep it free to enhance our overall value proposition."
Atom won't be closed source, but it won't be open source either. It will be somewhere inbetween, making it easy for us to charge for Atom while still making the source available under a restrictive license so you can see how everything works. We haven't finalized exactly how this will work yet. We will have full details ready for the official launch.
Where does it say it won't be free after the beta period?