Yes, Duryodhana and the Kauravas did go to Devaloka when they died.. This is because they died on the battlefield, which is the greatest death for a Kshatriya can have. Here is what Narada tells Yudhisthira in this chapter of the Swargarohanika Parva of the Mahabharata:
Here is king Duryodhana. He is worshipped with the gods by those righteous men and those foremost of kings who are now denizens of Heaven. By causing his body to be poured as a libation on the fire of battle, he has obtained the end that consists in attainment of the region for heroes. You and your brothers, who were veritable gods on Earth, were always persecuted by this one. Yet through his observance of Kshatriya practices he has attained to this region. This lord of Earth was not terrified in a situation fraught with terror.
This later chapter of the Swargarohanika Parva says the same thing about the rest of the Kauravas:
The sons of Dhritarashtra were all Rakshasas of fierce might. Sanctified by death caused by weapons, those high-souled beings of prosperity all succeeded in attaining to Heaven.