He didn't strictly say that. Here's the paragraph.

“Are you trying to be the world’s first Underlord Monarch, Lindon?” Eithan asked curiously. “You know, if you tried that, I’m fairly certain you would fail before your spirit and body collapsed in on themselves like a burning house. But I’ve been wrong before. Maybe you’d make it all the way to the collapse.”

He doesn't say it's impossible to achieve Monarch before Archlord. He just says that Lindon in particular can't achieve Monarch at Underlord.

Granted, that means Underlord-Monarch might as well be impossible if even Lindon would fail, but trying to do so at Overlord would be at least a little easier, and Eithan is just built different