Hi, unfortunately it is not true, in Seat Altea XL it is impossible to remove the upholstery without opening the flap, but the "gutter" can be reached through a specially made gap, just above the castle (photo attached).
We drive a screwdriver (or something similar) deep into the right side and move it to the left. Then from the hill: we unscrew two visible screws and two hidden under the flaps to the lights. We remove the upholstery from the latches and start having fun analyzing the problem.
For me it was clearly a poorly fastened beam, so it was done without additional costs (stupid is lucky).
And as always, thank you to elektroda.pl forum and its members for help!