It is.
This has nothing to do with hax or the haki-fruit debate: it's one of the shittiest decisions even made by Oda plot-wise and powerlevel wise regardless.
All Xebec's plot was about his defiance against the WG and now he got turned into a slave of Imu himself? The supreme leader of the people who wanted to slaughter his family? And all this, to shit?
Not to mention Imu could have done the same in EH; could do the fucking same in Erbaf and kill Luffy; could turn everyone, from Mihawk to Akainu, and kill everyone LOL.
Unless Xebec breaks free immediately, this is one of the shottiest decisions ever. Borderline offensive.