That letter is fake.  It was not know at the time, but Satoshi's mail account has been taken over -- no one knows how or when.  That was proved last September, when "Satoshi" sent a dox extorsion email from that address to some prominent bitcoiner.

Anyway, the phrase "in the face of widespread technical criticism and through the use of populist tactics" is a dead giveaway of its source.  I only cannot guess who wrote it because there are several people at Blockstream, and many more among its supporters, who would have used those words, would not have any scruples in sending a forged message to support their agenda, and are naive enough to think that the trick would work.

The second paragraph, moreover, is totally at odds with the very idea that makes the protocol work.