I see no reason why WP7 wouldn't support LTE. It's just that it will require a separate chip, because the Snapdragon MSM8x55 doesn't have LTE radios built in. AFAIK all current LTE phones have a separate chip for LTE. MSM8255 does have HSPA+ built-in, though, so I expect all T-Mobile/AT&T Mango phones to support it.
I think that article makes a good point, though. The carriers are all making a big deal about their 4G networks. They aren't going to put much effort into promoting/selling 3G phones anymore. WP7 desperately needs some 4G phones, because these are the phones the carriers want to sell.