Plug in plywood?

[bygges]

Craftsmen have been here and installed a vanity unit in the bathroom renovation. If I understood correctly, they drilled through the stone and adhesive into 15 mm plywood, then I saw that they clipped off the plugs and inserted them through the stone and adhesive into the plywood, after which they screwed with wood screws. They have had a business for 15+ years, but I still don't quite understand the reasoning behind it. Now the vanity unit is leaning about 5 mm at the front compared to the back, and there is flex in the unit. I "have learned" that plastic plugs are something you should avoid if you can, as it can end up just like this. Am I wrong, and why would it be a stronger fixation with plugs in the plywood compared to screwing directly, since that would be rigid?

 

[Krilleman]

There is no advantage.

Plyfa is not plugged, it is screwed directly.

Våtrumsinnervägg from säkervatten explains the procedure in a good and simple way.
https://www.sakervatten.se/bygga-ratt/byggforetag/vatrumsinnervagg

 

[BirgitS]

now the washstand tilts about 5 mm at the front compared to the back and there is flex in the washstand.

Sounds like something that can be claimed.