I don't think that issue was addressed anywhere before of Storm Front. Sources from previous editions would rather point at reasons for Renraku lack of relationships with Shinto.
Renraku used to be a megacorporation before being a Japanese corporation. It started as an Eastern European conglomerate called Keruba International that went global, before being taken over by Japanese investors, who renamed it Renraku Holdings. It took a while for Japanese executives to be appointed at every level (by 2061 in Corporate Download, only one out of five divisions was headed by a Japanese ; by 2071 in Corporate Guide, it was four out of five).
So, in spite of showcases like the Red Samurais (which were really just that in their heydays, shiny armors with no training and experience, before the management considered investing efforts and money), Renraku is, or was, a lot less entrenched in Japanese culture than Shiawase and MCT.
Regarding religion more specifically, Inazo Aneki retirement in Tibet in 2060, suggesting he personally was more into Buddhism than Shinto (at least at that point in his existence).