I am more saying that mentioning simultaneous launch and recovery in the first place as if it is a routine standard, is not a great look.
(Considering they say "Both of the catapults are situated close to the middle-front section of the landing area, so either the J-15 or J-35 (China’s two carrier-based fighter jets) would roll over the catapults when they land, temporarily preventing them from being used for launch operations and thus affecting the takeoff efficiency of the fighter jets," I am also unsure if that is or isn't factored into their 60% number)
Given CV-18 is a smaller ship, conventionally powered, with one fewer catapult and one (or two) fewer elevator/s, with a bigger island, it definitely should go without saying that its sortie rate will be lower than a contemporary US CVN, which I think any observer would have baked in as assumptions to begin with.
The rest comes down to design choices/parameters, airwing configuration etc.
Whether it's 60% or a different number, is largely immaterial, but a higher effort article would have had the foresight to account for size, configuration, powerplant, to actually squeeze out benefits or compromises of "changeable" design choices.