I would imagine it's about presence of an object-- i.e. when an object is specifically present, the ergative is triggered. With no object, then it's nom-abs.
I think it's rather that
ergative alignment is optional for siikhnaa. It's not at all uncommon colloquially to hear a
perfective siikhnaa without ergative alignment, even with an
explicitly specified direct object, and one finds plenty of literary examples on the internet as well, across the ages and across the "divide." Some further examples to add to the one in the OP:
Qayem Chandpuri (1725-1794)
siikhe ho kis_se sach kaho pyaare
ye(h) chaal-Dhaal...
Tell me truthfully, my dear, from whom have you learned these ways and manners...
Nazm Tababtai (1854-1933)
siikhaa huuN maikade meN
tariiq-e-farotanii...
I have learned the way of humility in the tavern...
Harishankar Parsai (1922-1995)
bachchoN, tum_ne baRe jaaduu nahiiN dekhe. chhoTe dekhe haiN to
chhoTe jaaduu hii
siikhe ho.
Children, you have not seen great magic! You've only seen little tricks, and so you've only learned little tricks.