No, they are not. The ratio of people over 65 years old is about 30% [1], with slight differences depending on which source you want to believe and in which year the statistics were gathered.

You could even aggregate the numbers yourself from the linked source in the article[2] (except their link has an extra space in it, so maybe use the one below). I was too lazy to do that, but I doubt it's too different.

[1] https://worldinmaps.com/country/japan/

[2] https://population.un.org/wpp/Download/Standard/Population/

The whitepaper actually says that 28% of Japan is over 65.

Maybe 75% is a misunderstanding of the "Old-age dependency ratios 1950-2050 in the 19 countries of study" chart. That shows that in 2050 Japan will have 74.32 people over 65 for every 100 people between 15-64.