What evidence do we have that this doesn't work with Secretkeeper? It has completely different wording than Bellringer Sentry, Mad Scientist, Mysterious Challenger, and Arcane Keysmith? In the past, Yogg-Saron, Hope's End was explained that it cast the spells, not the player, so that doesn't really apply here either? I don't know of a card that has this wording on it to compare?
Knowing Hearthstone wording though, it's just inconsistent keyword use.
Spells cast by other spells are not considered to be cast by the player, therefore no buffs. Example: Scroll of Wonder's spell does not buff mana wyrm.
The difference is that the mana wyrm only grows for spells played by the owner, while the Secretkeeper doesn't restrict who is playing the secret (it doesn't even mention the word player).
Still, we will only know for sure when we can play the card or Blizzard speaks out, as it wouldn't be the first time that the behaviour of a card is decided based on balance purposes.