David, you have just about convinced me. If you're right, why do you think history buried CBM's involvement?
These things happen. Wilson had been in charge of building the courses and had continued to develop and nurture them through two US Amateurs, until his untimely death. Through his years of study and hard work, he had become the person Merion most associated with the development of their courses, and for very good reason. In contrast, CBM's and Whigham's contribution had come very early in the process, before much if anything had been done on the land and their involvement would have been largely invisible except to those few who were actually involved in the design process. Out of sight, out of mind.
I don't think it was anything sinister back then by anyone back then. Wilson was effusive in his praise of CBM, and Robert Lesley thanked him and Whigham along with Wilson's committee. But his was at the dawn of a new era in the development of golf courses, and it was (and is) difficult to accurately characterize CBM/Whigham's contributions in a few words or general summary. CBM and Whigham weren't members of the Merion so they weren't on any of the Committees charged with creating the course. They were purely amateurs and so they weren't "hired" or "retained" by the club. The term "golf architect" was not really in common usage then, and when it was used it was applied more to professionals, and to those who were actually paid to build the golf course. So what were CBM and Whigham? Merion called them "advisors" which was accurate, but it made it easy for later generations to dismiss their contributions as being secondary or unimportant.
Also, the Wilson legend is a good story and obviously appealed to Merion's membership. Once a legend gets going it hard to ever set the record straight. One need look no further than the recent Golf Digest article to see how easy it is for a golf club to mistakenly credit one of their own members instead of properly acknowledging the contributions of outsiders.
Why did CBM himself stay silent about it?
It doesn't surprise me that CBM didn't mention Merion in his book. The book isn't anything close to a complete catalogue of CBM's contributions to golf architecture. For just one example, CBM doesn't even mention his redesign of Shinnecock Hills. If CBM had discussed each of his contributions to golf course architecture, it would have been an extremely long book.
As it was, only a few chapters were devoted to gca, and in those CBM focused on just a handful of his courses, and with these he had been in charge of
the entire creation process from beginning to end, including the construction of the golf course. I don't think CBM/Whigham had much of anything to do with the construction of Merion, so I am not surprised it wasn't discussed. He helped Merion plan it, told Merion what they needed to do, and left it to Merion to carry it out. So Merion was far from one of CBM's start-to-finish creations such as courses like NGLA, the Lido, Mid-Ocean, etc.
It might be worth noting that Hugh Wilson never took credit for the initial design, either. The closest he came to discussing the design process was when he praised CBM for teaching him what Merion should try to accomplish with their natural conditions. And in 1914 when Merion (through Robert Lesley) discussed the initial creation of the course, Merion credited Macdonald and Whigham alongside Wilson and his committee.
And when Merion's Board minutes indicated that Merion would build the course according to the layout plan approved by CBM and Whigham, there had yet to be a specific mention of Wilson's involvement in the design process.