On what grounds did Vladimir Putin invoke Article 51 of the UN Charter for self defence while going into Ukraine?

In his announcement (at 20:02) of the special military operations, Vladimir Putin invoked article 51 of the UN charter, justifying these operations as self-defence. However, since Russia was never attacked by Ukrainian shelling, which was being carried out in Donbas and the proclamation of independence of Donetsk and Lugansk People's Republic didn't make them part of Russia, how did Russia invoke article 51? Can other states invoke article 51 for states they are allied with?