Hmmm... Actually, I had never read that encyclical.
If I take out all the parenthetical clauses, I find this statement:
"This letter ... clearly points out that the first eleven chapters of Genesis ... do nevertheless pertain to history in a true sense in simple and metaphorical language adapted to the mentality of a people but little cultured, both state the principal truths which are fundamental for our salvation, and also give a popular description of the origin of the human race and the chosen people."
The Pope seems to be stating that the eleven chapters are netaphorical and that the truths contained are those that are fundamental for our salvation. Granted, it doesn't say the ONLY truths stated are those which are fundamental for our salvation, but that would be a strangely absolutist statement.
It also says that there is a second component: a "popular description of the origin of the human race and chosen people." That is a very funny way of asserting that something is true. If I say, "It's widely believed that..." do you presume that I am asserting the belief is correct? Likewise, "a popular description" seems to distance the Pope from a direct assertion. If he meant that this description were factual, would he not describe it as "widely known" rather than "a popular description?"
It seems to me that the Pope is trying to not conclusively state that the historical element (as opposed to the theological element) of the story MERELY myth, but he is plainly trying to de-emphasize the centrality of the claim that it is literally true in our objective, modern sense of truth.
Yes, the letter is criticizing not evolutionists, but those who discard the truth of Genesis entirely. But his assertion of what is the intended meaning of Genesis 1-11 is very limited.
As I understand it, any Christian can read it literally, and they might having been influenced by St, Basil's Hexaemeron but one is a liberty not to also.