It's not particularly hard to dynamically darken part of a pair of glasses.
What information do you have that this is “not particularly hard”?
As for what information I have, I could link you some press releases from FlexEnable, but how about I simply point out that you can slap a commodity grayscale LCD on the outside of any pair of glasses, whether or not they have an AR projector.
There are multiple brands of sunglasses and AR glasses that can already do global tinting with the press of a button, you just need to split that up.
The hardest part is getting the alignment right, but you already did the hard alignment work if you build AR glasses. Also making things as light as possible is hard but glasses that are a bit heavy are still valid solutions.
If you are starting at a display which displays a video feed (VR/MR form factor), you are correct.