I think it is fair to say. I've got experience playtesting non-core products for WotC. I had always wanted to playtest a core product, because there's more than one round of playtesting and communication.
Basic math should be one of the first things done. There's not enough rigor there. Different DMs are on different pages, diluting the value of their playtesting. It's also aggravating to see old errors that were previously fixed getting unfixed. That's the problem with reinventing the wheel.
The last public playtest document has already been released. People outside WotC have this one last chance to influence the game, and ...
Even if that happens, WotC can't whip up a new set of rules and say "is this okay? Good? Terrible? Did it fix the issue? Can you test them?" My playtesting experience informs me about this. Usually WotC will add extra material after the playtest, so playtesters look at the final product and say "how did X get in there? It's horrible. I never saw it. I could have helped fix that."
I agree with your basic point: getting the math right is really important. (Especially the note about monsters in 4e vs 3e is something I have come to realize myself).
At the same time, it's not THE most important part. Having perfect math doesn't help if the system is too clunky. Or too shallow. Which is part of what the wanted to find out in the play tests. 5e looks to be pretty fast, not clunky and not shallow.
4e had some "balance" problems as well. The basic "math" of of skill challenges and higher level play didn't work out. They added the "expertise" feats and their kind (defense feats) to make the math work. A shame in my opinion. The game would have been better without it. In addition, 4e is "clunky". Fights last a too long without fiddling with the system. Higher level play suffers from too many conditions. Stun/Blind/Daze galore. So, a game that prioritized math, got it slightly wrong and ended up being much to clunky.
So, yeah, the math is really important and I really hope they manage to get it right in 5e. That it wasn't priortized higher in the play test is understandable in my opinion.