Well, I tried to cover that already by writing "is considered to be incorrect". This forum is about LT, so I would think "... to be incorrect according to LT" was redundant.
LT is telling me "23 kilograms or less" may be incorrect. Kilograms is countable, so I should use "or fewer".
When I write "23 kilogram or less", then there is no complaint.
But when I write "I can bring 23 kilogram per suitcase", then LT is telling me I may want to use kilograms.
Try it yourself on https://languagetool.org/
There seems to be some kind of inconsistency. I'm sorry I cannot be more specific; English is not my native language and I only recently discovered LT.