I didn't say that the indexes would improve performance only that table variables CAN have indexes, you just have to create them when declaring the table variable. The optimizer will normally treat a table variable as having only one row regardless of the actual number of rows contained in the table variable. This is why performance degrades when using them as the volume of data increases.

The reason I say normally is I seem to recall reading somewhere that this is not always the case but I don't remember in what circumstances this could change and I haven't researched more on my own at this time.