I still say no, but there are those that say he is. The problem is that the only lore that correlates them as the same is the book on the Religious Varities in Tamriel. In the book, it loosely ties them together. Basically all the information that says they are the same being is from the non-canon information of Michael Kirkbride.
Yes, so for everyone else this means, if you perform a dim reading of a very few texts in a very large body of them, fail to draw educated conclusions and then ignore the explanatory material that could help you, listen to Sleign.
I say that they are different entities but could be correlated in some way, like he could be some dark aspect created when Nirn was finished being created, but he isn't some over dramatization of godhood that Akatosh can be countless people simultaneously.
And while we're on the topic of ignorance, we come across a related word which is ignore. Which is what you may choose to do in regard to the many references to contradictory, composite identity in the Elder Scrolls mythos.
From the top:
There is Time. There is a God of Time.
Both unquestionably exist. This is is certain because the various mortal races are descended from the et'ada, beings most of you know as the Nine Divines.
Elves and men have difference conceptions of who their most powerful and revered ancestor was. Mer see Auriel, Imperials see Akatosh, Nords see Alduin.
Because the process of creation was essentially a giant battle that took place in the absence of linear time, and involved more-or-less the death of the creator spirits and their re-animation as the gods we know today, and because these gods rely on mortal worship for power and identity, the different experiences of mortal cultures in the Dawn Era formed different divine 'personalities.' It may be helpful to think of Akatosh and Alduin as each being a God of Time, even though in reality there is simply one God of Time with multiple identities. They can all be active at the same time because there isn't an actual physical dragon sitting on a cloud the whole time.
Edit: And the Sheogorath/Jygallag dynamic was simply a curse switching personalities around in a Jekyll Hyde sort of way. The various identities of the Time God are all very authentic, constant states of being and it is perfectly natural, at least in the improbable, unstable realm that is Nirn.