I think because Saif has heard Konstantin in other venues and has seen how dishonest of a broker he is. His actions bely his protestations that he is not pro-genocide.
The debate he moderated with Brianna Joy Gray and Eli Lake was especially revealing: https://youtu.be/_M15lTZ41dM?si=prBKnHdbDrfOAbwK
Saif is in a difficult position of calling bullshit on someone who feigns objective impartiality but who is really a propagandist with an agenda.