It's ironic because Liam Gallagher was a much better singer than Harrison.
Yes, Liam Gallagher is almost always remembered for how his voice was destroyed after 2000, but we forget that during his golden era (1994–1997) he sang better than the vast majority of the Beatles.
If we get strict about the vocal side of things, outside of the group's famous harmonies, Liam was a better singer than all of them except Paul. He even clearly surpassed John Lennon himself who ultimately was his biggest influence and the direct origin of his style.
Lennon was a giant because of the emotional grit he put into it, but his technique was very rustic and he would end up with his throat completely wrecked on tours. In contrast, Liam had a physical build and a vocal apparatus that were a total rarity in British music.
Where did that difference come from? Pure anatomical design. The guy was born with an enormous ribcage and unusually proportioned paranasal sinuses. That gave him mask resonance and brutal volume projection: the air came shooting out with tremendous pressure, allowing him to achieve that metallic, bright timbre that cut through any wall of distorted guitars live without needing to hurt his vocal cords.
On top of that, he had a truly intimidating chest voice range. While John used more grit and George sang with a pretty limited and fragile natural tessitura which made his chest voice thin and lacking projection Liam pushed a massive stream of vocal power using pure ribcage.
He could sustain notes with a force and power in pure chest voice that neither Harrison nor Lennon would have been able to sustain in a stadium without running out of breath or cracking their voice, leaving George monumentally behind in that aspect.
And what about George Harrison in general? Here we have to draw the line carefully.
Vocally, the gap is abysmal: Harrison had a very pleasant voice for his quiet songs, but he lacked muscle and fell short on stage, sounding quite fragile. In fact, even Noel is a much better singer than George by a long shot, having far more control, pitch accuracy, and presence in front of the microphone.
However, in terms of musicality, ingenuity, and creativity, George was an absolute genius and Liam will never even come close to him. Instrumental genius, spiritual depth, the introduction of the sitar, and masterpieces like Here Comes the Sun or Something put Harrison on an unsurpassable creative level, leaving the Gallaghers eating dust when it comes to songwriting.
Is Liam an unbearable guy? Without a doubt. But the reality is that, separating Harrison's artistic genius which is indisputable with a microphone in hand and singing live, Liam Gallagher's performance, physical power, and chest register left George far behind.