I've read Humani Generis before, and I've recommended it to people who are really struggling with reconciling science and faith.
All that being said, not to play the "intent" argument, but I doubt Pius XII intended for people to be able to believe matter "intelligently organized itself." I suspect if you asked him, he'd say at least you have to believe God designed the process.
I do not doubt this, but unfortunately it needed to be said. That's why I call it a mistake and a blunder ... because he didn't explicitly put the proper constraints on it ... rather than a positive error.
He did something similar with NFP. He "opened the door" to it. He did not really intend to teach it, but his speech to the Midwives entailed some obvious (and unnecessary) public speculation. Prior to that, the Holy Office instructed that the subject must remain in the Confessional and only entertained with the greatest prudence and under all the necessary conditions. Again, Pius XII did not put the proper constraints on it, but just left the restrictions to some "grave", i.e. "serious" or "non-trivial" consideration, which has gradually been explained away and eroded ... to the point that we now have NFP being used as Catholic birth control. See, the Church has ALWAYS been very careful about opening doors to this kind of thing, anticipating that when you give people an inch, most of them are going to take the proverbial mile, and the Church always tried to cut that off at the pass before it could lead to this.
On both the subject of evolution and with regard to NFP, Pope Pius XII "opened the door." It's also the case that Pius XII during his long reign appointed the vast majority of Bishops who would eventually bring us Vatican II, so that's also on him. He really needed to find orthodox, staunch anti-Modernist priests to appoint as bishops and not this gaggle of scoundrels, 90% of whom were open Modernists. He was also the one who set up Bugnini with his liturgical experimentations (John XXIII actually got rid of the guy) and allowed other experimentation like "Mass of the Future". Pius XII also allowed various Ecuмenical conferences. Finally, he failed to condemned the heresies of Cardinal Cushing in the dispute with Fr. Feeney, and that led to a license to undermine and reject EENS and promote religious indifferentism (as Cushing did). Pius XII's was the watershed papacy that led to Vatican II. Oh, yes, lest I forgot, he also failed to consecrate Russia to Our Lady's Immaculate Heart with all the bishops of the world, and I am convinced that such a consecration would have prevented Vatican II. With all this he's responsible for, I would not be surprised if he's still in Purgatory, and will be until the end of time. That is why Pope St. Pius X wept at being elected pope, because he knew the gravity of the responsibility.
There's an interesting story about Pope Pius IX making a similar mistake. There were a couple statements of his in Magisterial docuмents that the enemies of the orthodox went with an tried to use to undermine EENS (the same ones that are routinely quoted ... and misrepresented ... by the anti-Feeneyites today). He was absolutely shocked (and angry) that people were interpreting the teaching that way and he responded with some strong statements reinforcing EENS dogma.