any contractor that tells you to place hardwood flooring on top of existing tile is a big hint for you to stay away from that contractor. That is not the best way to lay hardwood floor and it is the easy lazy way of doing wood floor.
Tile surfaces are uneven (for example grout line and no tile surfaces are even). If you want it done right, take off the old tile, buy the self leveling liquid to level out any imperfection in the concrete subfloor. No I am not a contractor or floor expert but many years of watching this old house makes me sounds like experts
So basically I just repeated what post #6 and #8 above mentioned.