The idea that an interview with a journalist would have any magisterial authority or significance at all is just laughable, regardless of what website it was posted on or how long it stayed there.
The problem is that Pope Francis is publishing it in book form (in an anthology of papal interviews). He clearly intends to promote his statements to a wider audience.
Well it must have some significance to the Pope who authorized the interview to be published on the Vatican’s website.
By the way, what has Francis proclaimed that has any “magisterial authority”?
Cardinal Burke has even questioned the magisterial authority of Evangelli Gaudium.
But it can give evidence of public heresy.