According to the scan, Superman, by his own words, didn't break free from the grip. The enemy let go because Superman punched it in the jaw as instructed when he was about to be eaten. Doctor occult is ambiguous on the reason why such a grip came apart, even Superman is surprised.
Also, grip strength doesn't mean you are applying that amount of force on the object when you hold it. If a being with a star-level grip holds a banana to eat it, it will not apply star-level force on the banana. If the object being held resists the hold, then grip strength starts to come into play. The banana can push back mountain-level force, and the hand will grip back with equal force to keep the hold. The hand can go up to a star-level grip, but that doesn't mean star-level force is being applied to the object in hand.
As shown in the scans, Superman's push against the grip < the Monster's applied strength.
Considering this is Year 1 Superman, who will legitimately show limits lower in Tier 6 in years 2-5, I consider the glass jaw hypothesis.