I dont' want to continue with this debate only because I haven't read much into it; But I have a few questions, 1- Did any of Imam Asharis students or students students say it is his last work?
2 - If it is his last work how does it differ with the early 'Ashari school?
And don't discuss 'Aynayn as that's rejected even if some of the early Ashari scholars said it for there's no Nass and it's been refuted by Ibn alJawzi, Ibn 'Aqil and Ibn Hazm.
3 - If you accept this work, then you have accept Ibn Kullab, unless you can provide differences between Ibn Kullab's position to those mentioned in Ibanah.
And to clarify to abulayl, Asharis don't deny the work, they say it's been tampered with.