This is true. However, the International Rules of the Road were in force in many countries prior to 1972/1977. There was the Motorboat Act that applied from some number of years prior. The Rules that you're referring to were part of the process that minimized or eliminated conflicting wording and made our US Inland Rules more in conformity with the International Rules and changed things like 65' limits to 20 meters and such. The basic concepts of General Prudence and Special Circumstances and 'in extremis' were around and generally codified prior to 1900 amongst most western countries. The Rules you mention were primarily a restatement of those rules with some streamlining and aligning.

Btw, while the USCG considers the Strait of Georgia as being Inland for the purposes of seatime requirements, the reality is that there is no such thing as Inland vs International with regard to Steering and Sailing rules and lights, in most waters outside of the US. Canada only has International rules (at least as far as this concept goes, I'm not sure about their portion of the Great Lakes)