Our definitions are just different. WWII shooters was a saturated genre. DOTA games are not. There's 2 of them that have any relevance at all.
Just like there are 2 (well, 3 now though I don't think MoH's long-term prospects are good) modern shooters that have any relevance at all Nobody mentions any other modern shooters anymore, all the comparison is between these. As such, how many games came before is basically irrelevant, it's the number of games currently in the picture that counts.
Yes, if you look at the absolute numbers there are many more modern shooters that have been released compared to DotA-style games. But if you are a new player trying to get into the modern shooter genre your options are Modern Warfare 2, Bad Company 2, and Medal of Honor. You'd never look at anything else. Likewise for a new playing going into DotA - they'll have LoL, HoN, and for what it's worth, Demigod.
It's never the total number of games in the genre that have ever been released that's important when deciding on a new game of the genre. It's how many current games in that genre people are playing that their new game will have to compete with. Apparently, Valve thinks they can create enough appeal for DotA 2 (probably by name, as much as anything else) to make it a successful competitor to the existing games. Time will tell if they manage