Is SQL Server's fill strategy the same for multi-file tempdb as multi-file (non-tempdb) filegroups?
Does this same fill strategy apply to tempdb?


Yes it does.

To prove it to yourself you can do a test similar to one described here:

SQL Server Proportional Fill Algorithm Example

i.e. populate temp table with some amount of data and check space used for every tempdb file.

I cite here only the conclusion:


  File groups use proportional fill strategy across all the files within
  each file group and uses a round robin algorithm to write a
  proportional amount to the free space in the file and to each file
  within the file group.
  
  Understanding the proportional fill and round robin behavior in SQL
  Server is important not only for user databases, but especially in the
  case of tempdb as well. The SQL Server best practice is to have
  multiple tempdb data files to alleviate tempdb PFS and SGAM
  contention. It is easy to imagine when the tempdb data files sizes are
  uneven, then the extent allocations would be heavier on the bigger
  sized data files. This configuration would again cause a hotspot and
  not alleviate tempdb contention.


And here is a further reading:
Correctly adding data files to tempdb by Paul Randal

Among other things, Paul describes how to add files correctly to match existing file sizes, and the reason of doing so is proportional fill algorithm.