I do not know what gardening centers are telling you this, but these can be applied now. Beneficial nematodes are in healthy soils year 'round. I would not pour a batch out during freezing weather, but we are not there yet!
It is best not to apply them when it is real hot. If outside temperature is above 86 degrees, apply in the evening. Garrett Juice is best applied in the morning. It is especially good to apply when humidity is high, even during a light mist as the "pores" of leaves are more open at this time.