The problem with USSR and General Daud AUN was that there was a difference of 4 years between his death and the 21 October coup. General Daud AUN was in the nation during the 1964 Border wars, and so it would have not made sense that his death was pre planned with that much of a gap.
His death was stated during "Military Training". What does that mean, did he die during training or whilst training. There are some topics missing, and it requires deeper reading about it. Why go to Moscow after the war? He was commander in chief after the war , and had trained with the Italians before independence, so why Moscow? It requires further reading and deep critical thinking in order to make sense.
But there is a small issue with the Wikipedia page of General Daud
Siad wasn't President at the time. So that means Aden Adde AUN was President at the time, and not Siad.
Had the USSR been planning the coup, which I think they were, then it was in motion for years.
I remember reading somewhere that Daud's death was deemed mysterious, I think it was from illness. This was in spite of his young age and reputed fitness.
As I said, the USSR was in charge of training the Somali military during the SYL years and it is very well known that Gabeyere and Barre were their point men in the military. If the USSR was planning the coup, it makes sense to get rid of Barre's superior in mysterious circumstance in order to allow Barre to assume the position that would enable him to take over the country.