There is no ambiguity there. You will get an economy seat or better. If non-premium seats available you will have an option to select one for free. If not, they will assign a premium economy or a comfort+ seat at the time of check-in. If the flight is overbooked having a preselected seat does not guarantee you won't be bumped.

This is something I've observed with different airlines. Google's line "Free seat selection" doesn't guarantee availability of seats to choose from, just that you won't be charged for selecting one if available for your fare.