Just got an email about the TOS updates which included:

“We want to let you know about few changes: (1) We’re adding a new functionality - GOG Direct to Account that will enable partnered platforms to activate, directly on your GOG.COM account, content purchased via these selected third party platforms;”

Sounds like they’re readying a method to sell keys on third party stores... good move... better exposure, more buying options.

So does this mean the GOG games will sometimes be cheaper if bought via the third party sites as opposed to on GOG.com?

Not keys, but yes a means for GOG games to be sold on third party sites without having to use keys.

Pretty much what Steam did with OAuth on humble for a while, which they stopped for unknown reasons.