I can't speak for Roku Engineering, though I'll be curious to see their answer.
I can tell you, however, that I had to look into this a bit last week. I made two copies of an M3U8 that were exactly the same apart from the #EXT-X-VERSION line, one set to 3 and the other set to 5. My Roku 2 happily played the #EXT-X-VERSION:3 and refused to play the #EXT-X-VERSION:5.