You keep referring to "TV" operated. But Transavia's IATA designator code is either "TO" (French operated flights) or "HV" (Dutch operated flights). I assume this was a HV operated flight, as it was flying AGP-AMS.

First of all, your knees thank you for getting to fly on a regular KL flight, and not on HV :-). HV is essentially a low-cost carrier, though owned by Air France/KLM, they operate completely seperate and are not a Skyteam member. KL, AF and DL do use a limited amount of their flights in their network to be able to offer connecting itineraries to airports not serviced by mainline, but only by Transavia.

Having said all that, something like this should of course never happen. You were ticketed on that flight, which means you should be allowed to board. However, I do wonder why this was booked through an agent and why the HV flight was chosen, as this is a route that is also flown by mainline KL.