It is true that Nicholas II abdicated on his favour and, in that sense, he could have been considered the last Tsar.
But, and that is a big BUT, he did not properly accept the throne.
In return he issued a manifesto under which he stated that a general election should take place and the government that would come from these elections would choose the form of government.
Michael would accept to reign as a constitutional monarch if the Duma would choose to keep the monarchy or move down if a republic would be installed.
So Michael neither accepted the powers passed to him by Nicholas, nor abdicated. He just postponed his position until the future elections that would clarify the political system.