To put it in terms that my little brain understands.......
It's because the .177 is a smaller, lighter pellet, it needs a bigger hit of air behind it to achieve a faster speed so that can make the same power.
Weight x velocity = ftlb energy.
Also, as others have said, the guy at the rfd is talking coblers .177 is fine for hunting
I have used .22 - my favourite rifle and and, my single honest keeper is my .22 TX.
For hunting, personally, I much prefer .177 - the lighter, faster pellet and flatter trajectory works for me.
I also like a heavier pellet - JSB Heavy 10.34gn. I'