If you take $50 as the base price, Valve take $20-$25 as their % of the sales price for selling through Steam (30% has been bandied about, but I think that is a special deal for indies and Valve take 40%-50% of AAA titles).
If you buy through a retail outlet, the sellers take at least $30 as their % of the mark up. Retail stores have to spend more than digital downloads on such things as a shop, then there's the middlemen and warehousing that the physical products need to move through.
That's the point that particular poster was trying to make, just didn't do it very well.
Buying through Steam would be cheaper if it weren't for price fixing forced by the retail stores. Personally, I'd be giving the retail stores the finger by releasing a AAA title that is cheaper on Steam due to the lower costs.
As for piracy, the key is to prevent pre-launch piracy. Generally it takes about 3 days to crack a title and free it from any DRM measures. HL2 was the first title for a long time that wasn't cracked until after it's release thanks to very robust encryption.