Share with anyone with the link setting is grey out, why?

Works fine here. Don't forget that the sharing settings can be configured on the individual SC level, so even if the org-wide settings allow anonymous sharing overall, some SC/ODFB libraries might not have the option available.

In my case, I can share just fine from my personal ODFB or from a generic SPO site. I get the same behavior as you when I try to share from a Comm site or Group/Team one. When I look at the settings of the corresponding SC though, I see this:

SharingCapability                        : ExistingExternalUserSharingOnly

So it seems working as expected.