In another place in the same Iblaagiyya Alf, Ali Muhammad Shirazi, the Bab writes:

'And Your Hujjat (Proof) is alive, the entire creation is sustained through his existence. Everyone and everything remembers him (Imam Mahdi). (O Allah!) You protect him in his Ghaybat and hasten his days (of reappearance). And I give witness that today Your Proof is (none other than) Mohammed bin Hasan, salutations be upon him and those who adhere to him.'

Conclusion
Here are some deductions from the above statement of the Bab:

1. Bab confesses that Imam Mahdi is alive. Importantly, alive at the time of the Mahdi, which means that the Imam was born much before the Bab.
   
   

2. Not just alive, rather he is the source of sustenance for the entire creation. This is an important part of the Shia belief wherein the Imam of the time is the means for all bounties that reach the world and heavens. In other words if it weren't for Imam Mahdi, the world and all its inhabitants would perish immediately. This is exactly what the Shia traditions say - a moment devoid of the divine Hujjat can lead to instant annihilation of the world and its inhabitants.
   
   

3. The world and it creatures getting sustenance through Imam Mahdi is a pointer to another important fact that Bahais have ignored all along. If Bab indeed was the Imam Mahdi as he was to claim later it only explains the world's survival for the brief period that he was alive - what about the interval that lasted over several centuries from Imam Hasan Askari's demise to Bab's birth? Who was the divine Hujjat during these centuries who ensured the survival of the world and its inhabitants including Bab?
   
   

4. Bab beseeches Allah to protect Imam Mahdi in Ghaybat.
   
   

5. Bab beseeches Allah to hasten the reappearance of Imam Mahdi. This fact is particularly significant for the Bahais. Imam Mahdi's reappearance was anxiously anticipated by the people since his Ghaybat. Bab accepts this fact. Therefore for him to claim Imam Mahdi's reappearance in his own persona a few years later while he was always apparent to the people hardly counts as a reappearance. To reappear you must first go into Ghaybat.
   
   

6. A big nail in the coffin as far as Bab's claim of being the Imam Mahdi is concerned comes in the form of his swearing to Allah that Mohammed bin Hasan (12th Imam of the Shias) is the Proof (Hujjat) of the time. This obviously means that Bab cannot be the Proof and Imam Mahdi for there cannot be two persons for the same position.