Well it's not actually that far off the truth. They are saying that there are times when the bettas have a lot of water, and times when the bettas do not have a lot of water. Stagnant does not mean dirty, only still.
As to Betta Basics, I used it in my uncycled tanks as my pH kept crashing due to my water being extremely soft. It did manage to hold the pH steady at between 6.8-7 for the three or so days between water changes.
Other than that it does basically everything that Seachem Prime does, just at a higher price.