Did Microsoft change query optimizer regarding number of files and parallelism
That was an urban legend that Microsoft's Bob Dorr debunked back in 2007:


  This information has been taken out of context and propagated
  incorrectly as SQL Server has a thread per database file so adding
  more files to a database can increase I/O performance.   Adding more
  files and properly aligning them with storage can increase the I/O
  performance for a variety of reasons but achieving new I/O threads per
  file is NOT one of them.


Read the post for much more detail.