Gday
Interesting.
I have an original Unitrode datasheet, and a quick search brings up the current texas one http://focus.ti.com/docs/prod/folders/print/l293d.html
Neither of these specify a 5KHz limit.
They mention doing their rise/fall time measurements at 5KHz, but thats all.
Based on the actual rise/fall times ( your link vs texas link ), they appear to be close, so i am at a loss to understand the difference.
I certainly have run my 293D at higher than 5KHz before.
Dunno