I hate Spanish' Baroquism in literature, a legacy from the Golden Era. 90% of these are pompous snobs.
Btw, 'he visto' would be fine... if the time from his childhood was stretched over.
I mean, here with 'he visto' it's referring both the childhood times and, indirectly, the present ones, as if he wanted to note that he saw something related to his whole lifetime.
Altough Borges would need to state better, -ya he visto- (I've already seen).
Oddly and poetically, 'he (tengo/have) vistos' would be far better (I have these -there- as already seen ).
