As I have said before, I just don't know if Mr. Mann spoke with police in 1975, or at any time after that. I am pretty certain that he was never interviewed by any press reporters until 2005 when his story was reported on the 30th anniversary of the girls' disappearance.
As you point out, he would be the only adult witness to have seen the girls that afternoon, since all other known witnesses were young teens.
If there is any kind of record of Mr. Mann making a statement in 1975, it would be a very strong piece of evidence. Unfortunately, if there is no such record, then investigators are left with a story based on 30 year old memories.
Richard,
It seems strange , if he did indeed tell LE , that he was never mentioned in any news reports. Yet minor children's accounts ( the boys who say they saw the sisters ) were mentioned in news accounts. That seems odd...
And if Mr. Mann never said anything at the time, why didn't he ? Wonder if he knew the girls by sight ? Or if they knew him enough to say hi to ? Wonder if he lived alone at that time ? JMO.