It says in SA OC Siman 124 Sif 11 that one may answer Amen to a bracha even if they didn’t hear the bracha (only because they know the bracha is being said and as long as they know *which* bracha is being said.)
What about in Kaddish. Must someone know to which “Amen” they are answering to of Kaddish in order to answer Amen. Or may one say Amen as long as they know the Kaddish is being said and hear when everyone else is answering?
The idea behind the halachah you cite is that answering amen without knowing what one is answering amen for is meaningless.
Therefore, the same principle will apply to the question of answering amen for Kaddish.