No, I am afraid you are misinformed. Bethesda has NEVER announced anything of the sort. It was information Del Rey accidentally published in the reviewer's edition of the book, and was removed from the final print. I believe the information Amazon and Waterstone used in the book's synopsis came from Del Rey.
That was not Bethesda, but a previewer, whose word can not be taken one way or the other, especially considering they took the comment down.
Yeah I looked into the statement about it spanning the time between the games. While that means that it may not be hard evidence, it is still pretty solid. I mean the publisher must've heard that from somebody. I think its still evidence, just not as strong.
Plus there is only really three choices: TES V happens before
The Infernal City, after
The Infernal City, Or perhaps during the plot of it. I think after sounds most reasonable. I'm sure that Bethesda is probably using
The Infernal City's plot to set up the plot of the next game. Do any significant changes take place in Tamriel as a whole by the end of the novel? Because if so thats probably what the novels were meant to accomplish, and thusly why they had the novels made. They probably had a good plot set up and didn't have the background of the main story established and created the framework of
The Infernal City to set up the plot elements needed. Oter companies release a gameboy game, but a book seems far more cool IMO.