I wouldn't call this not breaking a sweat.
He's quite clearly sweating, a lot at that given Chinjao doesn't even have a single drop of sweat on him in comparison.
Not to mention that's an insanely long clash for a quick flashback (Clash was spread throughout 5+ panels). There is absolutely nothing casual about that clash, gritting teeth, sweating, yelling, heck Garp even trained for this. All signs show a legit struggle in that moment.
Regardless I don't get the point of lying and saying he didn't break a sweat when he quite literally did just that?