Done it before on my own stuff and never had a problem. Just because something isn't the noŕm doesn't mean it's wrong. The description on that grit clearly states it can go onto ceramics tiles. I've messed around testing and it will even go on glass and you can then plaster on that too. Any old school traditional spread would say knock off and start again. They may also say that flexi trowel are crap and to stick with a carbon steel unit. I know the opinion on flexi trowel is another debate but it proves that methods can change.
I’ve plastered over tiles before and it has been fine but personally I would advise to remove tiles, make good and skim. Ultimately i think it makes a better job. If you do end up plastering over the tiles make sure they are thoroughly cleaned and free of grease etc before applying the pre-grit.