https://drafts.csswg.org/css-display/#run-in-layout

A run-in box with display: flow inlinifies its contents.

I don't understand how you can have a run-in box at all if <display-outside> isn't run-in ?
( display: flow is short for display: inline flow is it not?)
Maybe you meant "... when <display-inside> = flow" instead?
If so, shouldn't <display-inside> = flow-root run-ins also inlinify its contents?
(i.e. display: run-in flow-root)

https://drafts.csswg.org/css-display/#the-display-properties