Yes, there would be a bias. The phantom fanbase is, in fact, potentially larger than the 4E crowd, and plays Pathfinder. They want to attract those people. The open playtest has been widely marketed and I hope many of them are taking part, but if the numbers don't reflect the distribution of the potential market, you'll only be catering to your existing market. They shouldn't give the finger to the 90%, but they should consider their views at 50% strength, if they genuinely want to attract the other crowd.
There are several problems here.
The first is the possibility of an edition actually flopping rather than merely not performing to expectations. Losing the 4e fan base could be catastrophic.
The second is brand loyalty. Pathfinder players already have a game they are happy with, that is in print, and that is smart and agile enough to countermove whenever WotC looks ready to put out a new edition. Right now, saying "There are a lot of Pathfinder players out there - we should go after them" is like Pepsi's marketing division saying "There are a lot of coke drinkers out there. We should go after them."
The third is a mature vs an immature game. When 4e was first published it had limited options. More than it appeared but still very limited. Pathfinder is
drowning in options even before you break out books like the Book of Nine Swords from 3.5 and throw them onto the pile.
The third is brand disloyalty. Every person with Pathfinder has already seen WotC's work. It's going to be hard work.
Pathfinder players are not an untapped market. And WotC are not as good as Paizo at what Paizo do. They especially aren't as good at it with a new product as Paizo with years of experience, foreknowledge of the playtest, and who are already talking about a Pathfinder 2 (and they will have a vision unlike WotC who seem to be producing D&D as designed by the world's largest committee).
Going after Pathfinder players is hard work. And pissing off 4e players leaves WotC potentially ending up with no one. I've been playtesting 5e but the more I see, the more I'm looking at other systems. Taking Paizo on on the territory they've claimed is a Hail Mary pass, pure and simple. And one WotC need to telegraph hard, giving Paizo literally years to prepare a response.