Again the burden of proof is being unreasonably shifted. The letter came from the Holy Office to the Abp. of Boston. It was made public in 1953 and appeared in several periodicals. It was included in Denzinger. The document says His Holiness approved the doctrine set forth therein. The Pope is the head of the Holy Office. We have met our burden of proof. If anyone believes that Pius XII did NOT approve this, the burden of proof is now on him to prove it. Not on us.
Burden of proof as to what? That is infallible? No, you concede not. That it is free from error? No. That is a letter from the Holy Office to a bishop that has been published expressing views on a matter - most of us conceded that.
It's not the ordinary, universal magisterium teaching. It's a letter to a bishop, who was told (3 years or so later) that he could publish the letter. Great. That sounds like a person or some authority (knowing the story behind this affair, likely for political reasons) wanting to let an impression or position out into the public "unofficially" to serve various (again likely political) purposes, while allowing deniability later.
If they can deny it later, we can certainly question it now, albeit respectfully and as good Catholics ready to assent if the authority does later officially adopt and teach the position.
It doesn't require assent in its unofficial status, and even if it did, we could still question its reasoning.