Mikhail was technically Tsar for a matter of hours between Nicholas's abdication in favour of him and the point at which he informed the Provisional Government that he would only accept the throne if offered it by the people. Very technically Tsar - for most of that period he did not know he was Tsar!
Equally, Alexei was technically Tsar in the interval between Nicholas's two abdications.
It is rather like Lady Jane Grey. She may have been technically Queen under the terms of Edward VI's Devise for the Succession (which is of doubtful validity, since Edward was a minor and under a regency), but certainly not a monarch in any meaningful sense.
Ann