It's related but not the same. Entrenchment reifies positions, whatever they may be. Polarization is closer associated with party values and perhaps extremism. But that's mostly my guess as to how the author is interpreting these effects. The ideological backgrounds they assigned were random, so they may or may not have been a fit for whoever they were assigned to. What the author may be observing is just that the memes have an impact when people happen to already agree with their statements, and that this impact was not necessarily along party lines.

I'd double check the full article to find the discussion on this if you can - I tried but it's not yet available through my institution.