I would say directly ripping XI's races, using the core "every class on one character" concept, and implementing a number of other XI terminology and battle concepts, is enough to make most consider XIV a "sequel" to XI
The similarity in races was to ease and encourage the transition.

What battle concepts might those be?

And playing multiple classes on one character is in no way a 'sequel' to XI. Please see: Final Fantasy V, Final Fantasy X, Final Fantasy Tactics, Chocobo's Dungeon, and probably others that I've forgotten.

Also, Tanaka.