I have found no indication besides the recipe above(and copies thereof) that the method that boils or steeps the nettles works as a rennet substitute the way thistle or yellow bedstraw do.
The handful of commercial operations producing nettle cheese specify that they are making a lactic set cheese using the lactic acid bacteria on the nettles.
In that case, boiling them would be counter-productive to say the least.
From my own experiments, I believe that the effect of the plant itself, if any, is very minor and only happens after the lactic acid bacteria have lowered the pH significantly 24+ hours later.
I got the best set from raw nettle salt 1:1 paste aiming for around 1-2% salt to milk. Decoction+inoculation did also eventually coagulate but the strength didn't matter much.
I think that the salt is there to favor lactic acid bacteria over other kinds.