Just having a burner phone works against dragnet surveillance if one is not doing really stupid shit like logging in to one's personal social media accounts from one.

If however it's an actual crime which actually gets investigated by actual criminal investigators, they're going to be coming at it individually and using much more specific techniques than just "use a surveillance warrant to get a list of all mobile phones that connected to certain cell towers at certain points in time and plonk them all on a database to cross-check with similar data from other demonstrations".

You can't just treat a burner phone as a second phone that you have active anywhere near your home, place of work or places you normally frequent and you can't just keep it and keep on using it for a long period of time: the longer one holds on to that burner phone the more data points there will be that can be bulk checked with other, identifyable, data from other sources (say, car tracking data) to find out a more than normal overlap.

I wouldn't at all be surprised if those people with the burner phones had them with them active whilst ridding their personal vehicles which had something like OnStar or were dumb enough to log-in to their Facebook account from them.