Pity it won't work for chocolate holograms.
This technique with laser seems to produces the diffraction grating by varying oxide layer thickness not by creating 3d texture so resulting surface is still flat and attempting to use it as mold will not transfer the pattern to chocolate.
The reason many commercially available diffraction gratings have 3d texture (and thus suitable for copying with chocolate) is because stamping a hot piece of metal into plastic is a very cheap way of doing it.
Anyway, there are still ways of moving forward with the idea. For example, chemically removing the oxide layer to a desired thickness sounds feasible. If I were him, I would try it (but maybe in another video, as the whole process would be a whole different rollercoaster).
Instead, you could choose a different metal whose oxides are easy to etch. Magnesium is probably the extreme case here, with an oxide that instantly vanishes in the weakest of acids, but if someone gave me a US$7000 fiber laser, I would try to keep the laser beam away from thin pieces of magnesium. But mild steel, for example, forms oxides that etch pretty easily with acids. I think copper oxides also etch easily with either acids or bases, too, and the copper itself is more resistant to etching.
Really, though, if you're molding silicone or chocolate, you don't need the high strength, flexibility, conductivity, etc., of metals. Maybe etch your grating into a material chosen for other properties. Glass, for example, is perfectly isotropic and has no grain structure to introduce into your cuts, and it has a low TCE. It sticks to silicone, but not to chocolate. Fused quartz is a glass with a near-zero TCE. I assume but don't know that the MOPA laser can ablate the glass surface.
Other amorphous solids might be more amenable to easy laser shaping and not stick to silicone. Sugar glass, for example.
Is there any risk of the laser going through the glass and damaging parts of the machine? Or things such as refraction?
As I understand, those machines have a sheet of materials they can work with. This is probably the safest way to go: just stick with the materials the laser is made to safely operate on.
Otherwise, it's skim an edible oxide layer over the chocolate to etch.