There's no doubt Warren has been doing better lately. It's also true that it's a fairly modest surge. The RCP rolling average has her at about 10 percent of Dem primary voters, not really different than where she was 3 weeks ago, after which she went down a bit and then up a bit. But it's reasonable to think of her as being in third place behind Sanders, who has stalled out, and Biden, who has lost a bit of altitude.
She's also not running that much better than she has been against Trump in trial heats; Biden continues to lap the field in this regard. In the latest Quinnipiac poll, Biden crushes Trump by 13 points, including, astonishingly almost tying Trump among white voters. Warren beats Trump by about half of Biden's margin, with about twice Biden's deficit among white noncollege voters. How she would fare among this demographic in a general election continues to worry me.
(Below Warren is the brown line, when you click thru to the full picture)