For this specific situation, it absolutely works. Here is Dispel Magic:
Choose any creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends. For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a successful check, the spell ends.
So you can choose the creature who has cast the spell, CHECK.
It is a actual spell we are trying to dispel, CHECK.
Readying a spell:
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration (explained in chapter 10).If your concentration is broken, the spell dissipates without taking effect.
A spell has been cast, CHECK.
There is magic energy an it is vulnerable to dissipation/dispel by numerous means. CHECK.
The spell has been cast, so it is a done deal, the energy has been activated (slot used) and you are holding the spell energy within the caster. There is no way around this very specific fact, by specific over general RAW, the spell has been cast before readying it. There is an active spell, it is vulnerable to being dispelled/dissipated and losing the already cast spell (and the activated spell slot) by other means, those being loss of concentration or holding on for too long, and reaching your next turn.
The readied spell completely qualifies for the Dispel Magic spell.
This has nothing to do with readying a Dispel Magic spell and using it, this is the other way around, do not confuse the different situations.
The Sage Advice, already quoted about that different situation, is completely correct, in that you cannot dispel a spell not already cast. However, in this instance the target has specifically cast the spell and the magic energy is active.
To quote the most pertinent part of the ruling:
The main reason is that dispel magic removes a spell that is already on a target, whether that target is a creature, an object, or some other phenomenon. Dispel magic can’t pre-dispel something. If a spell isn’t already present on a target, dispel magic does nothing to that target.
The target is the spell caster and there is a cast and active spell present on them in the form of the active spell they have readied. By Sage Advice's own added conditions, the readied spell completely conforms and is vulnerable to the Dispel Magic.