Maybe when it comes to the Presidential popular vote (which only wins them the electoral college a little less than three quarters of the time), but nowhere else.
Basically this. The ~2.5% advantage doesn't matter if you don't get to control anything with a narrow "win." Dems "won" by 0.5% in 2000 and then by 2.1% in 2016 and both times came away with nothing to show for it federally. 5/8 in actual EC wins isn't historically impressive, especially when 2 of the 5 wins came during economic crises. And then there was the 1% win in the US House in 2012 that didn't even come close to a majority, and don't even get me started about the state legislatures.
It's probably just an artifact of presidential/legislative candidates not actively competing for the PV to begin with. When conservative downballot candidates actively compete in Safe Dem states, they tend to outperform the presidential losses significantly.