There's tape to be used with most backing boards as well as the glue
What are they? They look similar to Marmox & joints have to be taped with that.
This sort of stuff https://nassboards.co.uk/product/self-adhesive-waterproof-jointing-tape-5-meters/#&gid=1&pid=1
Check the fitting instructions for the sheets, they should tell you if taping is required
The BBA docs for those backer boards will be based on them being fitted to manufacturers specifications rather than to the design of a pair of randoms on Youtube. So read them, do what they say & if you run into problems down the line you'll have somewhere to go with your complaint.
Just a little heads-up- waterproof tile adhesive is not waterproof. It should maintain adhesion under wet conditions but that isn't the same thing at all.
EDIT If you're fitting 100 bathrooms then saving £50 on tape on each job is £5k in your bin & you're long gone if any problems show up. On one bathroom it doesn't stack up as well...
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