If you want to split hairs about exact terminology, then /reasons/casus belli/. I hope we don't disagree that their "denazification" story is bullshit.
Are the 10 biolabs that were built in Ukraine close to the border with Russia, starting in 2014 - with the explicit purpose of researching gain of function on coronaviruses and targetted genotypes as part of the CIA's PREDICT program - also bullshit, in your opinion?
Because, in spite of your opinion, its not bullshit.
What would the USA do if Mexico or Canada were suddenly sprouting up clandestine bio-warfare laboratories, funded by Russia or China, within miles of its border? What would Israel do if Iran had funded similar biolabs on its borders with Egypt or Jordan?
That's very subjective. No one starts a war for bullshit reasons. They may sound bullshit for you but not for them.
PS: hopefully I don't have to point out that _reasons_ and "denazification" casus belli are different things.