It's essentially a forced loan to the government at subpar rates. The "tax" is the delta between what the government pays out for the bonds vs what a bond of equivalent risk in the free market would have paid.
The magnitude of the investment also probably makes it impractical for anyone but the very wealthy to retire before that starts paying out. Most other countries have lower rates on their retirement schemes, which makes it feasible for more people to live on their savings for a few years before the government retirement scheme kicks in. E.g. in the US it's pretty feasible for the upper middle/lower upper classes to retire a few years before Social Security kicks in, especially if they're willing to live frugally.