Hi Paul Stawski With local breakout for the Office 365 endpoints in Japan you should hit the local front door for SharePoint Online and also the CDN content Microsoft publishes (scripts/generic images etc), regardless of where your tenant resides.
This doesnt use DNS but an Anycast address to find the nearest SPO front door which should be in Japan near your egress. SharePoint CDN as per the link you sent, can be used if you wish to host elements of your SharePoint pages within a CDN as opposed to within SPO itself. In which case it may help with performance by delivering them locally. Not all customers/pages require this additional step so i'd start first with checking performance with a local breakout in Japan for the required endpoints.
Regards,
Paul