The ABYC standards are partly derived from the CFR for boat builders, which is mandatory, and partly from evidence based practices so some of what is in the ABYC standards is mandatory if you are a boat builder.
Well, here's what CFR Title 46 Shipping, subpart C part 120 Electrical Installations says;
(g) Each connection to a conductor or terminal part of a conductor must be made within an enclosure and have either:
(1) A pressure type connector on each conductor;
(2) A solder lug on each conductor;
(3) A splice made with a pressure type connector to a flexible lead or conductor; or
(4) A splice that is soldered, brazed, or welded to a flexible lead or conductor.
Yes factory soldered joints are almost always fine as they are done to a standard. DIY soldered joints are most often not done to a level that can even compare to a wave soldered device.
I was talking about things like accessory connectors provided by manufacturer's with their equipment where it is up to the installer to provide the necessary cable and install these connectors by soldering the wiring to the pins of the connectors using a soldering iron. These are not done using wave soldering techniques such as is done with printed circuit boards.
I agree that the average DIY does not have the knowledge/skill/experience to do marine wiring properly and in accordance with established standards and laws. I am a marine electronics service technician and installer with 34 years of experience (you have one year on me). I have seen it all as well, and not just DIY's, but by so called expert professional's.