The author is using a DLP projector to backlight the LCD. When the projector is displaying black it effectively is infinite contrast. The light is reflected away and never reaches the LCD to backlight it.
Have you seen those samsung qdoleds? In a normally lit room they look more washed out than a VA panel, yet somehow still having an "inf" contrast...
On the other hand, there's very little light spill from DLP--- and what little there is, is attenuated by a really big factor by the LCD.
I'd guess the limiting factor is light scattered elsewhere in the room and by the front surface of the display.
What he is making here is a dual panel display. There are actually some LCD monitors that use this concept of 2 separate panel layers each providing their normal contrast resulting in a multiplicative effect.
For the video posted here, the DLP is probably at best 1000:1 and the cheap old LCD is also maybe about 1000:1, so you get about 1 million:1 which is a lot, but not technically infinite like self-emissive display that can turn off the pixels completely for sections of the picture like OLED and microLED.
I would say the weak point of DLP is the screen material needing to be light-friendly, but that's on a sliding scale with the emissive display, not a hard line difference. Both of them preserve black levels when turning on, unlike traditional LCD, but also it isn't true black for either of them.
For example the Epson 5050UB or LS12000 do about 5000:1 native contrast. Most DLPs around the same price level are in the 1000:1 to at most 2000:1 contrast level.
A DLP is nothing like an emissive display. The entire area of a projected image with any projector is always lit up by the light source. Any projector, including DLP is like a TV with a single backlight and no local array dimming.
Here are some simple photos of DLP projectors and here you will clearly see that the black parts, like the black bars above and below the cinemascope image, or even the spill over past the edges of the panel are not black at all.
https://i.redd.it/ak3lwsumvccc1.jpeg
https://preview.redd.it/eyc62402amnd1.jpg?width=4000&format=...
In that second photo, the odd shape of the black projected area is due to the user using keystone, so the image is not being displayed on that part of the imaging panel and it's just left black, but you can see then what the black level really is for projected "black" pixels.
How is there so much light going the wrong way from a mirror?
If DLP has worst contrast than LCD, then why would this project be even remotely successful? Wouldn't the LCD layer in the stock LCD TV would have already been more effectively blocking the light from the stock backlight?
So yes, if the internal light engine of the DLP truly reflected all light away, then, if it outputs zero photons for full-screen black, you could say the projector outputs zero photons.
However, the DLP is not 100% effective; the interesting images will have a mix of bright and dark areas, and the lens and software image processing will raise the black levels above zero.
Source: I worked at BrightSide Technologies before and after the acquisition by Dolby Laboratories and learned about DLP lens limitations from the engineer working on DLP+LCD based HDR displays.
Contrast is important, but alone it is useless. You could have a display with only two levels: black and white, and if the black is perfectly black, then it's infinite contrast (any level of white, no matter how dim, is infinitely brighter than total black), but 1-bit color would be useless in reality for home theater.
To make infinite contrast useful, it's necessary to have HDR as well to take advantage of having very dark and very bright areas both able to be represented with detail. Plus having sufficiently bright maximum brightness so that the brightest highlights can really shine.
SDR benefits incredibly as well. HDR is another elevation.
Getting stuck with LCD contrast for years was a travesty to image quality across the board.