It doesn't matter if she was referring to her personal books or those in the store.
Waterstones sell books.
Her job is to sell books.
Telling her circa 36k followers a book by a GC author is only worthy of the bin is contrary to the job. Then extrapolate that into presumably all books by GC authors are bin worthy.
We don't know how many of the authors books Waterstones have in stock and how much £££ in sales she put at risk and/or risk of legal action by the author if any deal has been done about promoting the work.
It may be quite small scale wrt to this specific author, but had Waterstones not acted what happens if she posts similar about the next book she takes aim at that turns of to be a bestseller - everywhere apart from Waterstones as GC authors refuse to have their books stocked there.
Hell, I'm betting Waterstones will have a big pre-order in on the next Strike novel.
She knows she messed up. She says it's my first mistake. Note: mistake. Her mistake and it's a biggie.