@Tactless if you put it that way, the author himself has been lamenting and depressed for months, and this manga is also a form of recuperation for him.
for those that is already aware (author's closed circle of friends & family) the whole manga is the period of "somberness" you mentioned.
But, I guess for those that arent aware yet (most of us), we will find it a bit sudden. Even though there is a warning that the death is coming in 100 days, it is still pretty sudden.
But personally I am happy for the author, on how he chose to move on and honor his best friend, and I hope his anime will be received warmly