I usually use merck vet manual for myself.. but sometimes reference a dumbed down source if you will forgive the use of that expression to make communication easier..
https://www.merckvetmanual.com/poultry/ectoparasites/mites-of-poultry
The Merck manual does indicate that the SLM can be transmitted by contact so MAYBE they do come to the surface to be effected by the permethrin. But it also states.. to spray premises and treat the bird with ivermectin or sulphur solution, the follow is quoted from the link above..
"For control, affected birds should be culled or isolated, and
houses should be
cleaned and sprayed frequently, as recommended for the
poultry red mite.
Individual birds should be treated with oral or topical ivermectin or moxidectin (0.2 mg/kg), 10% sulphur solution, or 0.5% sodium fluoride."
So your effective SLM treatment was with permethrin only or also the use of vaseline?
The previous reasoning for inefficacy being the same reason it was considered ineffective against depluming mites is they may not ever come to the surface.
Permethin applied specifically.. is even effective even against termites. Some areas have seen over use and reduced efficacy like with anything else (including Ivermectin and even spinoasad).. it (permethrin) worked really well at controlling chicken lice/mires for me.. but won't touch flea populations on my dogs (brought by a visitor), neither would Frontline which worked well in my previous locations.. requiring use of more expensive prescription drugs.
While we're on this topic.. I wish to mention that Permethrin is NOT safe for use on cats. I still use it despite having cats.. just making sure it's dried completely before they approach and not on their stuff.
I always appreciate a valid and informational discussion.. I'm here to share and to learn not to be right.. and boy how my understanding (and practice) has changed through out the years.
One final statement.. I don't prefer to use Ivermectin on my birds as it is off label, egg withdrawal times are not well studied, and all the blah blah blah.. where as the permethrin is labeled for use in poultry with NO egg withdrawal time.
Something funny.. a post from you and other on this exact subject ..
https://www.backyardchickens.com/threads/permethrin-spray-for-scaly-leg-mites.1320760/
Regarding resources.. I also often use the state University stuff noting it doesn't always reflect real life experience.. this one is Iowa's..
https://store.extension.iastate.edu/product/15670.pdf
I value personal experience to! it turns out the authorities and other powers that be don't always give us entirely accurate or valid information, thank you for sharing and questioning!