The 25 foot setback being referred to is for buildings, not septic beds.
You cannot build any structure closer than 25 feet from the shore line.
that is only common sense. Lake levels do vary according to precipitation and ice floes could cause damage to a structure placed right at the shore.
From your own website:
“This property is outside of a mandatory building inspection area, meaning all you have to do to be legal is maintain the proper setback from the lake”
Septic beds must be kept at least 100 feet away from a well or source of water used in the building. Depending on the percolation rate of that specific ground this may have to be increased.