This is incorrect.
The Magic Stone cantrip is not triggered by a ranged spell attack. It allows you to use a bonus action to create a small supply of magical stones that you can use to attack people using the mechanic of a ranged spell attack. The "ranged spell attack" in the cantrip description is the act of using the stone you created at casting time. It has nothing to do with the casting of another spell.
You could cast the cantrip as a bonus action, and then use your Action to make a ranged spell attack and throw one of the stones or to cast Eldritch Blast.
However, you could not both cast Eldritch Blast and throw a stone in a single turn. It might be possible if you double classed with a second class that grants you a second attack, since throwing or shooting the stone is a "ranged spell attack" and not "casting a spell". This is not the case with a straight 11th level warlock, though.
This is upheld by a question asked of Jeremy Crawford, presumably by someone here, since the wording is so similar.
The conversation goes back and forth for a bit, but the pertinent quotes are:
Jeremy Crawford: Eldritch blast doesn't hurl objects
Jeremy Crawford:The act of hurling a magic stone is the ranged spell attack.