Compare that to the patent application diagram which shows U bends fitted to both circuits.
View attachment 115029
That is a large number of soldered joints to be made on an automated production line. No doubt the quality control is high and as a result the production yeald is high enough to make the design cost effective. Failure rate of installed boilers is very low and thus the joints appear to be reliable in service.
The complexity of the manufacturing the Intergas heat exchanger seems high compared to the tube in a tube construction of the Ferroli heat exchanger.