I don't know where you came up with that bit of "wisdom"......... I pitch harvested yeast directly all the time, sometimes months old...... no problem at all. H.W.
Ultimately, the wisdom comes from those in the know zone.
Harvested yeast, whether it be days old or months old is not a magic panacea.
Dependent on the beer to be fermented next, the amount of harvested yeast, whether it be days old or months old, might need a starter to get to proper pitch amount.
I can provide this wisdom..
I have been harvesting yeast now for close to 2 yrs with the help of Jamil and Chris White and Woodland and their literature.
I have dialed my process in pretty well and get really clean yeast.
The ONLY time that I do not make a starter is when I direct pitch on top of an existing yeast cake.
I would NEVER pitch a slurry that is a couple of months old without making a starter, even if I had a proper amount of slurry.
At the very least, you want to wake these little critters up and get them ready for the task at hand.
I am not saying that your way won't work, I am saying that you may not be pitching enough yeast to do the best job that they can.