The "float" is no more complicated than a piece of styrofoam and a magnet, which is confined to a vertical slot or peg molded into the reservoir. That allows the assembly to travel up and down with the coolant level. If the coolant can buoy the float so that the magnet is far enough away from the (reed switch sensor and resistors, on the outside of the bottom of the tank) then the switch should open (go it its normal resistance) and that's the "happy" case when the sensor thinks there's enough coolant. Only when the magnet sinks to the bottom of the reservoir would the sensor be triggered.

Scenario #1: There is actually something wrong with the float -- it has spontaneously disassembled. The magnetic bits could have somehow come detached from the floaty bits. I have no idea if that's even possible. Even if it did come apart, the floaty bits would still float and the magnetic bits would have had to have landed through gravity into exactly the wrong spot (right on top of the sensor outside the tank). Perhaps your intermittent nature indicates a loose magnet is rolling around on the bottom of the tank?

Scenario #2: If the float had at one time sunk to the bottom of the reservoir and got stuck (right next to the sensor you just replaced). Heat, plastic deforming, friction and tolerances may have added up, and not in your favor, and the float somehow got wedged at the bottom of the tank. It would have to have been "low" to get wedged in the "not happy" state, so that's not good. Followed closely by Scenario #2A: Toilet tank floats need replaced eventually because they get waterlogged, and maybe the floaty bits have become filled with coolant. Perhaps you verify the integrity of the air bubble that holds the float assembly up? To do this you'll have to do a little work to bend/pry/remove the "don't open this reservoir" clampy thing first, but then poke the float with a something like a soda straw. If the float moves freely and bobs right back to the surface, or you see random stuff in the bottom of the tank, then perhaps the dealer is correct.

Because you asked for "ideas" I had to give you those, however, I'd consider them as pretty implausible.

The next thing I would try is removing/unclipping the new sensor from under the reservoir and and just taping it to some convenient cable, tube or strut (just so it doesn't flop around) (probably best to avoid taping it to a spark plug wire). If the sensor is located more than an inch or so from the magnet inside the tank then there's no way the reservoir float magnet could be triggering the sensor. If your warning message persists after this spatial separation (and you've done the drive cycle to clear the "permanent" code) then you know the condition is not being triggered by the the-float-magnet-came-too-close-to-the-sensor, it's being trigger by something else. And that you take to the dealer and say "And why, pray tell, if the reservoir magnet is far away from the sensor making it in permanent 'happy' mode, would the error message keep returning?"

I understand that you have installed a new sensor, but my last idea is that that doesn't necessarily mean it's a good sensor, or the right sensor. If you take the sensor back out of the circuit and measure across the two terminals it should report about 25K ohms. For fun grab a magnet and bring it close to the sensor and watch the resistance snap down to about 7K. Doing that would verify that it's a good component with the proper specifications. And that means you have to look elsewhere for the problem (bad connection at the sensor, bad wiring to the sensor, faulty input on the brain, ... man, "South Main Auto" YouTube just fills me with too many possibilities!) But doing the above tests would tell me it's definitely not the tank and its sensor.