Just returned from two weeks at Aulani, Saw a fair bit of people with coolers of all sizes even on wheels. No issues at all and regarding bringing drinks again no issues as long as it was not in glass.
If this is an offical rule it's in the running for one of the top 10 never enforced rules at Disney.
I didn't know it was a rule and have never been questioned about my little Owner's Locker cooler. Heck I have seen folks with the giant rolling coolers and enough food/drink for the entire pool area
And just because I don't want to clean my house I went looking. Found this photo of the "pool rules" at CBR. No mention of coolers