The citybuses-like in quite a few other smaller FR cities, are all free-which is easy to see as if there are many pax to board, they use any entrance. As also very often in FR, these are for an ´agglo´ which also includes a few outer areas/communes, who have opted to pay in for this system.
For those with a tipical british view: if one counts the total cost of printing, selling, keeping an office etc the tickets and the various in FR inevitable social low fares for special groups and the general very low level of busfares anyway it may well work out nearly as ´cheap´=the total cost to pay up for those communes as going free vs taking payment. It was the case in Paris for RATP too that the total ´perception cost´ for taking the fares was nearly as high as the total takings. But since then the general larger area over there has taken over.
Citybus system in nearby Dunkerque-Duinkerken/Dunkirk, DK-bus, is also free. Goes to the BE border to connect with flemish deLIJN buses to link to the coastal tramway. Used to be an international line till this free system came in.
The very few regional routes going from Calais to surrounding tiny villages are not free, but nearly so, I think these are just 1€/ride. On some special days and for early bookers the local TER-trains, of course still by SNCF, also offer 1€ cheap fares.
In general the french also very much like to make-up fancy names for their networks/routes/fare systems etc. I guess (website will probably show) what this Balad´in stands for (going on balade is going for a walk in /fr/)-it is not a ballad, as for song. System is called like citadin and for the opale network-this part of their coast along Channel is named ´cote opale´ in /fr/. Their own website comes in /fr/ only and clearly states, f you go all the way in the general info to find it for tickets/fares, that it is indeed gratuite since 2020.
As an aside for how low bus fares can work out: a yr or 2 ago I did a trip out from Toulouse ito the end of a branch line in the far south for about 80 KMs-for SNCF is was even a reduced special low fare of 10€ and coming back by various buses, some also free, was 1,60 total-but took more as twice the time.