Hogwash.
American Casualties only began to tail off noticeably the second half of 71 when Nixon's troop pullout really got some traction.
The number of Americans who died in 71 were similar to the numbers in 66. The numbers in 70 parallel those of 67. (You can check out the monthly figures for yourself.)
It's simple, American deaths went up during LBJ's troop buildup because there were more people in country each month. They went down because Nixon started withdrawals.
btw..my Vietnam service medal says we got mired there starting in 61 not 64.