For example, yellow has a wavelength of ~580nm. When that wavelength enters our eyes it activates both the M and the L cone cells, with the L slightly more. That differential activation between the two types of cells gives us the experience of yellow.

Blue actually activates all our cone cells, with S the most and a tiny bit of M and L.

What this technology does is selectively activate only the M cone cells.

There is no possible wavelength that activates only the M cone cells, because the combined activation spectrum of the S and L cone cells overlaps the entirely of the M spectrum. There's nothing natural that can enter our eyes and activate M without also activating L and/or S.

Therefore no one can ever have had the experience of having only their M cone cells activated with anything in the natural world. It can only have been done with this technology.

As for the experience of seeing olo, the qualia, that's almost arbitrary. The authors originally thought it was going to be the "greenest green" but it wasn't really. It wasn't possible to predict what the qualia would be, because the brain has never had that experience. For all they knew they might not have seen anything, because the brain could have just thrown up it's hands at this nonsense signal. Instead they happened to experience this teal-like color. But that wasn't a priori obvious. It's just the brain struggling to apply an experience to nonsense.

It is similar to tiring out the S and L cone cells and then looking away as you say elsewhere -- the authors mention that in the long-form article in The New Yorker -- but this is much clearer and more saturated.