Please do not confuse the "external sealing" of the windows with the base sealing. The latter may be done by the landscape gardener or tinsmith (in our case more likely the roofer). The external sealing of the windows primarily refers to the protection against driving rain and that is the responsibility of the FB. This is clearly not given here with the open foam insulation.
In my opinion, the threshold should also include the connection to the (appropriately prepared) base sealing. After all, the tiler does not omit the last few centimeters above the shower tray because the sealing tape from the plumber starts there.
Furthermore, the craftsman is liable for defects and a missing connection is undoubtedly one. If a planner is involved, he can possibly be brought in as a co-responsible party, but the craftsman cannot simply use this as an excuse to escape liability. Also: if the window fitter cannot execute the connection because other trades have botched or omitted it, he must report this before execution and have the client commission the necessary rework, or at least have the faulty execution “released.”
I am not entirely sure at this point, but I would claim that such a “release” can also be in the form of a contractual agreement that essentially states something like “lower sealing is explicitly not commissioned as this is done by the Spengler GmbH company.”
However, this probably would not exist with an “all-in-one general contractor.”
If everything had gone right in this case, the base sealing would have had overlapping EPDM sheets, to which the FB could simply have connected with his sealing tape, and everything would have been fine. Now it gets interesting.