Good advice if it’s an exhortation to get out activating frequently 
I suspect this is a myth carried over from experience with other Lithium Ion technologies. Lithium Iron Phosphate chemistry is much more resilient to storage at high levels of charge. Manufacturers generally warn of the danger of deep discharge which is death to the cells, some recommending long-term storage starting at 100% with regular checks (6 monthly) for this reason.
An issue with trying to store LiFePO4 batteries at part-charge is knowing how much charge is in them - with, in a typical cell, possibly less than 0.2V difference between 90% charge and 20% charge and the natural variations found between cells it can be very difficult to be sure of the actual state of charge of a partially discharged cell.
HTH
73 Paul G4MD