It's not undoubted. Both sides of the conflict say something different so it really depends on who the individual sides with.
Research briefing from The UK House of Commons Library titled "Conflict in Ukraine: A timeline (2014 – eve of 2022 invasion)"
Toward the end of February 2014, unidentified military figures, later confirmed to be Russian personnel, surrounded the airports in Crimea, a majority-Russian peninsula in Ukraine. The Crimean autonomous assembly was then seized by pro-Russian forces.
In March 2014 the assembly issued a declaration of independence and a subsequent referendum on union with Russia was held. According to Russian election officials, 95.5% of voters supported union with Russia. The results of that referendum are not internationally recognised.
Then on the United Nations website the March 2014 "UN general assembly voted not to recognize the referendum held in autonomous Crimea" - 100 votes in favor to 11 against.
So at least 100 UN countries are doubting the validity of the referendum.
I found the same account of events on the US Embassy website, Wikipedia, Britanica, and several North American news sites. My point is not to be argumentative but the messaging will be different on both sides of the conflict, as these things always are. So Zelensky isn't "lying" since he and at least 100 other nations believe what he's saying.