It does make a difference, the smaller s7 with a 8000mah battery has better battery life than the 10000mah S7+. The rest is just your speculation.
Apple specs claims
10 hours of battery life for
all their iPad devices, whether it's iPad mini 2, iPad Air, or iPad Pro 12.9". The 12.9" gets around 13 hours based on reviews. This is not a mistake on Apple's part. Once you have more than 10 hours, it's not important.
Galaxy Tab S7+ exceeds 12 hours, even with 120Hz. The iPad Pro 12.9" has a 8% larger battery.
So, no battery life was not the reason Apple declined to use Samsung 120Hz OLED panel.