I have a copy of that article too. The W124 is my favourite car from the '80's, number two would be the '89 BMW 5 series.
I've never heard of the BMW inline 6 story, but it might be true. I mean BMW and MB watch each other very closely. A good example is MB's introduction of the W201 in response to the 3 series dominance of the entry level market. BMW held the title of world's best inline 6 before and after the M103, M104 engines came and went. BMW didn't have any 8 cylinder engines until the introduction of the V8's in '93 or '94. This means BMW's success in the 1980's was powered entirely by inline 4's and 6's.
Mercedes-Benz's 2.8 liter DOHC inline 6 was a big disappointment in the US when used in the W116's and W123's. MB was not very successful at adding on the equipment needed to meet US emissions. While a Euro 280E had 177 hp, the US 280E was heavier and had 142 hp in federal trim (137 hp in California). MB dropped the 208E in the US for 1982, which left all US w123's diesel powered. The odds of MB having an all diesel E class in the US again are about as good as the odds of the Detroit Lions winning the SuperBowl anytime soon! So MB developed a 3.0 liter SOHC inline 6 that was universally better then it's predecessor. It was even physically smaller despite have a larger displacement.