I don't think that's true. Where are these useless spells coming from? You can maybe get one overlapping secret like you suggested, but most people will just refrain from playing spells while Cho is on the board and then kill him. Meanwhile, you've locked yourself out of playing spells unless you want to harm your position. Lorewalker Cho is a pretty horrible card.
Useless for them. A mill rogue deck has a bunch of bounce spells that are less useful for most other decks. Vanish, Sap, and Shadowstep are very useful for mill but not so useful for most other decks (with the exception of using sap on the deathlord). Gang up is also not very useful in non-mill decks (ganging up the cold light works because cold light makes both players draw cards, meaning a mill player will get their other cards faster).
Assassinate obviously will be useful for opponent, but if they use it before killing the cho it gives the mill player back their assassinate, which is why it's best to have cho in play with a taunt minion.
You say that having cho makes the mill player less likely to play spells, but why would I hesitate to put cards like vanish, sap, Shadowstep, and gang up into the other player's hand when it helps reduce how many cards from their deck they get? It's a similar reason that before I replaced it with cho, I would have king mukla in my mill deck. I replaced the mukla with a cho when it occurred to me that the aforementioned spells would be less useful to the opponent than bananas.
For reference, this is currently my mill rogue deck:
- 2 backstabs
- 2 shadowsteps
- 2 youthful brewmasters
- 2 saps
- 2 gang ups
- nat, the darkfisher
- lorewalker cho
- 2 eviscerate
- 2 cold light oracles
- 2 dancing swords
- 2 deathlords
- brann bronzebeard
- 1 earthen ring farseer
- 2 refreshment vendors
- 2 antique healbots
- 2 assassinates
- 2 vanish
The earthen ring is just there because mill rogue needs whatever healing it can get (it used to be 2 farseers before I decided to replace 1 with mukla, which then got replaced with cho). Same reason for the two vendors.