She directly stated she didn't use magic at all, implying the crab lock was all muscle. The immense magic reserves have nothing to do with this.
I feel you are getting lost in the context, completely disregarding the fact that she didn't have to put in the work. She's royalty, she's a woman, and she has a past life where he mastered magic. She had 3x the incentive to stay in her lane and live a relaxed life, but she disregarded all that and still chose the hardest road, attaining mastery in a completely new domain.
Also 'training with leisure' has little to do with it too, since even though she's royal the rest are still nobles, so it's not like they have to work and have little to no free time.
The lock is entertaining and entirely self-inflicted. I mean, this is the armored princess, we're talking about. (same to the principal since he's unintentionally aiding her cause, but I digress.)
Now, it's a comedy series, and the absurdity and unseriousness is part of its charm, which is why it's fun and easy to point out such ridiculous statements to begin with.
Also, did she have to put in the work, no, but that wouldn't give her mother any reason to faint, so can't have that...
Extended Response:
Considering the fact that she's capable of physical feats likely matching the hero, and that it's been widely stated that she is beyond human comprehension in terms of physical and magical strength, there's only so far you can go without natural born talent. She does have his bloodline.
If it was just effort, she wouldn't be able to overpower all those men who've spent their entire lives training for even a fraction of her physical strength, on top of never having magic, with just physical strength or otherwise. + No one is discounting how much she's accomplished besides her. Her lack of grace, inability to sit still, and her position allowing her to easily disregard almost all societal norms aside, most non-faceslap victims acknowledge her overwhelming achievements, just would rather her not bring it into places without consideration of the time and place.
Even this chapter was an example of her going beyond human limits, as explained by her cousin/admirer who I can only hope is actually able to get swole. Also, she's always using magic while training to keep her muscles subtle, which ironically makes a really good reason for her being absurdly strong...