Correct.
Nobody can force Ake to sign for Chelsea. All a buyback clause means is that we have to accept the bid from Chelsea at that fixed price, not that Ake has to sign for them. They could be the only club to bid for him and he could still turn it down.
Alternatively they could bid at £40 million and Arsenal also bid at a level we find acceptable, say at £50 million, and it's then up to him to decide which move he preferred.
Or Chelsea could choose just to not get involved at all and he just goes to the other club. All the buyout does is give Chelsea an option to make an offer that has to be accepted at a fixed price. Everything else about the transfer beyond that is normal.
There are then some complications on what that acceptable price other clubs would have to pay in order to stop them trying to pull a fast one in cooperation with Chelsea. So he is probably available at under true market price at the moment. The Telegraph suggested £45 million to other clubs but I'd hope we could push that a little further.
Edit: I'm sure there was a post I was replying to when I typed that! Maybe a 2020 hallucination...