Market Cap to GDP

Hi,

Why is market cap to GDP at 100% considered overvalued? I know the long term average is substantially below this, but just purely logically, if the market cap is equal to GDP, shouldn’t that mean fair value? Below 100 is undervalued, above 100 overvalued?

I guess:

Market cap of shares doesn’t take into account debt, so market cap should always be below GDP?

Does this make sense?