You made the assertion, you need a source. I’m asking why would it be, to me it doesn’t seem at all clearcut.
issur shmia is even when it was said ltoeles
issur shmia is even when it was said in front of others already
saying food at a restaurant is bad is lashon hara unless it is litoeles
Lashon hara includes speaking about a craftsman in another city where he does not work.
The owner would be more pained the more people heard it
the reputation becomes worse the more people heard it
I am lost as to how you are trying to thread the needle here with no source for the distinction you are making. This should be the same as any other of the above cases that are clearly assur.