Harry asks Draco if Draco had submitted him to torture with no intention of helping him. Draco is already under the effects of Veritaserum, so he'll testify.

Draco isn't even present. His earlier testimony under Veritaserum was simply read aloud.

And also the possibility of utilizing Draco's use of torture was mentioned and rejected in the previous chapter, as Lucius Malfoy may well have obliviated anything incriminating he found in Draco's mind under his own Veritaserum interrogation of his son.