Earlier quoted context omitted.
Presumably because it would not be deployed on them. Or they would have received assurances of a treatment/antidote being provided. Top $ paid to officials, military aid grants to the country, US security assurances. The research could be done by outside companies, just used Ukraine as a black site to get around existing international laws.
If biological weapon would not be deployed, then why Ukraine need it at all? There is no other hostile country populated by Slavs AND separated by ocean from Ukraine, to be a target for such weapon. Can you give us a logical reason why Ukraine needs a weapon for mass-suicide? Other nations may want a such weapon, but why Ukraine may want it to such extent to participate in development of such suicidal weapon?
Because it's part of Russias propaganda and disinformation campaign against Ukraine, which has heated up recently a head of the counter offensive because they know they are losing in that space.