Maybe in powerlifting that works but this is baseball which relies on the functional strength necessary to perform baseball moves. This includes a lot of various factors like proper mechanics, flexibility, general athleticism, etc. Gaining 40+ pounds is going to affect all of that stuff negatively.
Also, let's not pretend like Manoah lost so much weight that he's now skinny. The dude is still very chunky. Probably even considered obese on the BMI scale. He's just not shockingly fat anymore.
And finally, we saw what the all-powerful fat f*** version of Manoah could do and it was abysmal. He might as well try a season while having shed significant weight.