There are many tilers who have served their time who do not yet know that chipboard is not an ideal surface to tile to. As a result tilers still do it. Manufacturers also produce adhesives (usualy with admix) that are said to be suitable for over chipboard but, the honest truth is that it is risky due to the characteristics of this type of substrate.
There are some tilers who still think PVA is a great idea, untill a fe wyears ago people on this forum didnt even know that it was unwise to use PVA!
Tell your tiler to have a look on this forum to see whay it is no longer the choice of an 'informed' professional tiler to use PVA anymore.
In my experience it is those who do not take the time to research up to date industry
standards and regulations who continue to work to older and dated standards in tiling. They may not use the internet much or may be a small one man band who never take on work that has been specified by an architect etc.
With this in mind I wouldnt be too harsh on him, it may be purely down to him working this wy for so long and he doesnt know any better. I am very confiedent that many good tilers used pva at some point in the past.
AS for my opinion, I would choose to oversheet with ply or cement backer board prior to tiling. If a primer is required 9 times out of 10 I choose to use primer G from Mapie unless it is a specific surface.