Tiler trying to tile directly on top of a 9mm OSB sheet, using standard adhesive. Is this a good/bad

Recently started to renovate the flooring in the house - we had carpets and we want to get tiles. However as I read on the topic, I became a bit concerned about how the contractor is doing our flooring. The contractor is supplying all of the materials apart from the tiles themselves.

The old carpets were on top of a timber base, where some planks were loose. The tiler fixed the old planks into place, and added a substrate on top - a 9mm OSB, this one in particular: https://www.wickes.co.uk/Wickes-Gen...nd-Board-3-OSB-3---9-x-1200-x-2400mm/p/102972 They then bought some standard adhesive (but haven't used yet) https://www.wickes.co.uk/Mapei-Fast-Set-Ceramic-Tile-Adhesive-Grey-20kg/p/234986, and are planning to use them directly on the OSB subfloor. However, earlier today I saw in the product documentation they were "unsuitable" for wooden substrates, and another "flexible" one shoudl be used to lay on top of wood.

This concerned me a bit and so I checked through the rest of the set up. I found the majority of resources online are suggesting that laying tiles on OSB boards are a bad idea, and in any case the 9mm is too thin. Instead something like this should be laid instead to tile on top of: https://www.wickes.co.uk/NoMorePly-12mm-Fibre-Cement-Construction-Board-1200-x-800mm/p/225437

This did not look good to me of course and I am planning on brining this up with them tomorrow. As I'm not very familiar with flooring, I'm not 100% sure of my conclusions and so would really appreciate a second opinion on this! How big of a problem is this?