When I read 7-digit my first thought was this seems a bit short sighted. Then I saw the the address was going to be a combination of letters and numbers. Since this is the English page for The Japan Times I'll give them a pass on using the wrong word.
Letters are digits. Just not base 10.
Alphanumeric codes are mathematically speaking a completely identical set to base 36 numerics
I was looking for this comment as I thought 7 digits wasn't enough for the 55,000,000+ households in Japan. I missed the fact that it's alphanumeric - problem solved.
FWIW The Japan Times is a fully English language publication, so no need to excuse anything based on language ability