Odd. When I tab through the video while it's playing, the PiP feature does get announced by my screen reader (NVDA) but it cannot be activated by the keyboard.
Bug 1665251 Comment 1 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
I'm wrong. The pip button does show up in the tab order but I can't seem to activate it.