I'm a bit confused by this comment. The assembly instructions[0] clearly show screw holes on the back of the unit and it appears to even include a template for putting the screw holes into the wall you're mounting it to. It doesn't look like the holes would be suitable for mounting under a table though, they'd probably prefer that you replace the table with a STARKVIND.

[0] https://www.ikea.com/ca/en/assembly_instructions/foernuftig-...