Well, it's hard to say exactly what he meant, not knowing the context.

But generally speaking, most coaches seem to accept/know from experience that cyclisists can maintain peak racing form for a limited period of time- weeks. Part of the art of coaching is delivering your athletes to their most important races in peak form while helping to organize the rest of their season so that they do well enough in their easier races to learn, develop skills/fitness & remain motivated.

Trying to maintain peak form indefinitely is a recipe for disaster (ie overtraining).

If Friel is referring to "optimal" fitness as being peak form, then since you're at you're peak for a few weeks, there is a period of time you'd need to maintain it. But he might be referring to fitness that is optimal for some other purpose- for example, a level of fitness that is an optimal base fitness, which you could maintain by doing XYZ. It's hard to know without context.