I think this is the video he's talking about. (at ~4:20 )
The reason why he's wavedashing so much is because it puts you in a better state than when dash-dancing once the WD is over since you are free to do anything and dashing is still quite restrictive. Both have their pros and cons. While dash-dancing, you can turn around at any time whereas you are stuck in the sliding animation when doing a wavedash. So it makes sense to wavedash if you expect that you will not need to turn around so soon and it makes sense to dash-dance if you are expecting to have to react to something very quickly, like a whiff punish.
In this particular case, he was outside of m2k's immediate range most of the time so he wasn't vulnerable at the start of the wavedash and could enjoy the superior and more threatening position they put him in. However, he might not be ready to punish one of m2k's aerial if m2k guesses the timing of the wavedash, here a dashdance might have been better. The reason why hax is able to pull this off is because :
1. He is nearly frame perfect with
a. his wavedashes and
b. acting out of them immediately
2. He understands the commitment of each wavedash and what they mean in the context of the situation (whether they are meant to bait out a certain attack or threaten a certain range) and gains information about the opponent's state of mind from each interaction.
3. He knows what to react to and how to react to it after the wavedash. When you start the wavedash, you have to take a split second and really try and see what the opponent does and react accordingly. This takes a lot of time and effort to become proficient at. You can spend hours practicing certain scenarios and thinking about ways to act. Dr. Peepee speaks about this sort of stuff a lot in his marth thread (sticky in the marth character forum). Maybe check it out.
