It's about enforcement of laws and policy. No nation has "open borders". Porous is probably a better description. But it's about messaging so "open" is a simpler term.That still does not mean open borders.
You can criticize the decision to let them in but saying it is open borders is not accurate.
Considering Obama was the Deporter in Chief and build the kiddie cages I never saw the Dems as open border. Just quite hypocritical in their messaging. But the messaging is part of the problem. It encouraged people to try. And weaker enforcement due to optics(although I laughed when they used Obama era pictures of Kids in Cages to try to bust on Trump) and trying to please a vocal base just created more problems.
The citizens of the USA don't like to admit a portion of their economy has always relied on, dating back to slavery, with having a controlled minority to handle domestic work, farm work, and other menial tasks. Preferably one law enforcement is free to handle roughly to keep them in line.
Democrats, if they really believed in freedom, would roll back sanctions policy that drives people to seek asylum. And make laws that punish the employers of illegals. That would kill the market.
Of course then the rich fucks would have to pay fair wages, and the Dem donors don't want that either.