I don’t agree with “They don't control the demand for their power” because Bhutan do have choice over how much they deliver, even if it is only in the long term via their contracts. Bhutan exports 70% of their power, so clearly India wants it.
As an aside, I have two very good friends who work designing electricity markets, and planning country interlinks. I am not a specialist, but I do try to keep up with the play.
Without specific knowledge of the agreements between India and Bhutan, this is also hypothetical, and I have to say that it's a big assumption that "India clearly wants it." And even for economics 101 commodities, no supplier controls their demand curve or the optimal quantity to sell.
I get your points, but ultimately Bhutan has control over building hydrodams within its borders, so it is controlling supply, so it does have a lot of control over the price. Of course you can come up with a million quibbles about the details, but you are just arguing yourself into a corner case.
OP’s point was: “Are you suggesting they cannot sell more [electricity to india]?”. You said “They probably can't”. Yet Bhutan is developing more hydro dams, and they are going to sell more electricity to India. Case closed!
Most likely there is a treaty setting a floor price on the power or a demand ceiling, and they have hit one of those. Those limits are negotiated based on all of these considerations.