Are Calisthenics exercises really *that* compound, or am I doing something wrong?

Hey,

I am new on this subreddit. I started practicing Bodyweight Fitness a few months ago. I still consider myself a beginner, though I can proudly say I have already improved a lot.

I use some YouTube videos to design my workouts, but I mostly follow two books, one of them being Hybrid Calisthenics, by Hampton Liu - the gentle Asian guy with the black tank top, who also has a YT channel.
At the risk of doing something too easy for me, I always start with very basic variations of exercises and then move on to harder ones - I am very thorough.
Noticing I hadn't (ever) practiced leg raises, I started with the easiest Leg Raise exercise on the book: Knee Raises (in which you always keep your legs bent at 90 degrees). Hampton suggests I should be able to get to 30 reps of Knee Raises before moving to the harder variation: one in which the legs are partially straight.

Long story short: I can't get past twenty.
Are my abs too weak? Actually, no, I think I already have a relatively strong core: I can easily keep more than one minute of forearm plank, I am already learning the pseudo-planche lean, and even aesthetically, my six-pack is quite definite.
Nono, the problem isn't the core.
The problem is with the legs.

When I do Knee Raises, I feel like my calves are about to explode. And it is weird, because it shouldn't be an exercise for the legs, right? Yes, surely these exercises are compound, in the sense that even if they are ment to strengthen your core, they will also strengthen other parts of your body.
But still, this doesn't feel right.

I wondered whether I have abysmal chicken legs. But no, I can do thirty deep squats, twenty narrow deep squats and I am learning Bulgarian lunges. I can even do a few of Pistol Squats (though not with good form, so I left them out of my routine).

What is going on here?
Is it normal to feel such searing pain during an exercise that Hampton calls "too easy for most people"; and besides, even in such a part of the body?