I am bringing this up not as facts that Ukraine is building a dirty bomb, but as a fact that Russia has been talking about the possibility of them doing so for a long time now.

I read the parent as a report of what RT used to say in the past and may do so soon again to spin some legitimation for putin's upcoming war actions.

I think that OP does not believe in RT.com. Most probably, OP is bringing forward information that corroborates what's wrtitten in the Twitter thread. By reading the post's source, you may understand the intent of the parent comment.