When I read this I had a sinking feeling from it. This suggests to me that the feature is entirely optional; a cosmetic "let's please everyone" concession ~(to let them play that way if they want to). This is potentially bad. The problem with this (if this is how it is) is that it suggests that the map designs are likely indifferent to the chosen exploration mode, with no regard or use for the deductive aspect afforded by a fixed grid map. Players used to be able to use the grid to count off steps [precise distances] and compare what they've previously explored to find suspicious areas of the maps. This could then suggest likely places to carefully scrutinize for illusions, teleporters, triggers or secret passages. Most especially the ones needed to find hidden areas on different maps!Additionally, though the camera movement allows for and shows off free movement, we have laid the maps out such that you can snap into grid movement as well.
For instance one might see evidence of a hidden room or passage (or a fenced in room) with no obvious exit; this is where the player would start examining their maps, but also the maps above and below the room in question. This is practical when the maps themselves are not arbitrary, and do follow the grid. This allows the player a reasonable boundary inside which to puzzle things out; but this doesn't exist when the maps are laid out any-which way, and where one cannot be sure ~of where a thing should be, or cannot be. When the choice is "could be anywhere at all", nobody looks; or looks hard enough; and it's disheartening to know that there is possibly nothing there to be found by puzzling it out.
This is not a lament of free movement [per se], but is the assumption of the maps being just beautiful corridors [of forest or stone blocks]; where the path is direct, and everything is laid out, and on display. That would be quite a disappointment IMO. I've seen the video of the rocks reforming into a threshold; and that's what I mean by on display. If the map designers did not care in which mode the players explore, then it seems unlikely that they would devise aspects that would only become clear from careful observation and deduction with respect to the nature of grid based maps.
Something else comes to mind (and perhaps to consider)... When DreamForge released Menzoberranzan, the game had free movement and/or step based grid movement. However those responsible for testing the game did not spot (or did not fix) many places that unintentionally REQUIRED free movement to be traversable; most notably caves with stalagmites. These actually blocked passage to the player using grid-step; and for a novice player who wouldn't know better ~yet, these passages were indistinguishable from any other blocked passage, and so were not explored; later maps would prove impossible to complete until one realized that the game was messed up, and that you had to use free movement to get around stalagmites. So I do hope it's a certainty that the BT4 maps can be fully explored using either option; and I hope that [being a dungeoncrawler?] the BT4 maps are a challenge in and of themselves, outside of any monsters chosen to populate them.