Leandros should've reported the issue to the Chapter Chaplain which... I find hard to believe wasn't a thing on the planet since it was a considerable deployment of forces, both of the Ultramarines and later on even other Chapters. Plus the Inquisitor felt compelled to blackmail and threaten innocents (Lieutenant Mira, Sergeant Sidonius and the rest of the Ultramarine Company) if Captain Titus did not comply. I'm not sure how the Ultramarines feel about threatening obvious innocents, but there are other chapters that find such theatrics frown inducing behavior at best.
It's also wild seeing a pair of Black Templars are somehow functioning as the Inquisitors goonsquad. I can't imagine the Inquisitor being all that compelling in exacting effective authority over the Black Templars to open fire on the Ultramarines if they don't comply. They'd probably get intentionally distracted by taking the shuttle they just arrived on for a 'test flight' or something and leave the Inquisitor behind.