It's a weird comparison, but no if desalinated water is being delivered to taps you'd expect deslinated water to be cheaper than tap-water, if you take the price at the source instead of the price after a (not free) delivery process.

I think OP is saying that if desalinated water is being delivered to taps, then it is tap water.

I agree that's what OP is saying, I'm saying that's not true. The "is" operator here isn't commutative, "tap water is desalinated water" is true, but "desalinated water is tap water" is not.

Let's put some numbers to it. Assume tap water is desalinated, it costs $1 to desalinate a unit of water, and $1 to deliver a unit of water to taps. Then tap water is going to cost $2 while desalinated water only costs $1. Delivery to taps isn't free, so if you just want to buy bulk desalinated water it's cheaper.

I assume the implication was "cheaper than the current typical cost of non-desalinated tap water."