If you want to dig into this more, this paper has a lot of tables about wealth holdings (and what forms those holdings take) over time and at different strata https://www.nber.org/papers/w24085.

Edit: I think this is the relevant table: https://i.imgur.com/h8Fo8yx.png. If I'm reading it right, this is likely the source of that 84% number. Note the line "Stocks, directly indirectly owned", and the note that it includes retirement plans. I'm guessing this also includes pensions, since that would be a retirement plan. There are many interesting tables in this paper, however.