What am I missing here? Can’t you make every electric guitar sound darker by bleeding off high frequency content from the signal?
I’ve seen some of the greatest guitar players in the world dial back a tone pot a little to reduce brightness. You haven’t offered any actual explanation of why “you can’t make a bright guitar darker” is even a little bit true.
Is it possible he was only referring to the unplugged sound of the guitar, where modifications might make a dark unplugged guitar sound brighter (e.g. stiffer, heavier bridge), but not vice versa?
He didn’t post here. You did. I asked you to support the claim you repeated, but you haven’t.
Are you sure he didn’t say the reverse? I’ve always found it a lot easier to take something out of a sound that’s originally there, than to add something in that wasn’t there in the first place.
Yeah, boosting something that isn’t there isn’t really ideal. I’d have assumed he meant the reverse, which is straightforward in comparison.
Well technically, with passive electronics, you can make a pickup brighter or darker. Either way, it’ll still be lower output than no tone change at all because of passive electronics.
This is probably true on stock telecasters; the tone capacitor choice is more about taming the high end attack of picking, not to make it warmer. However, using a 0.1uF cap (if my memory is correct) will change the characteristic of the tone pot, allowing for a “warmer” sounding pickup.
This is all just assuming passive electronics tone changes only. Once you get into active electronics (and more “techy” solutions like @Fossegrim mentioned with EQ curve matching in a DAW), you can make all kinds of drastic changes, bordering on “identical sounding” tones.
…though, this is all a VERY long and technical discussion about something that may or may not be theoretically possible, but if you can get there simply by putting in a different set of pickups and get to a point where it’s plug and play, then that’s certainly easier. 
That’s what id do, and what I have done with two of my guitars. There are many things you can do after the fact, in a DAW, using exciters on di’s and reamping, eq matching in a similar way to cab ir’s, simply using a good eq plugin etc etc etc.
So many possibilities. I’d rather just plug in my guitar and have it sound the way I want without having to do complex workarounds to allow me to play with these changes, as opposed to just changing the sound after recording.
I did a lot of pain in the ass editing, eq-ing before reamping etc etc when I was doing studio work. It’s a lot of work and wouldn’t be necessary with the right pickups in the first place.