It can go either way.   You might not want to fully trust your product to OGL 1.0a.
Let's say I want to use a third party product.  I'll go with Runeblades again.  In it there is a runeblade called Twilight.  Twilight has mechanics attached to it and I really, really like it.  I can't use the name, because that's not OGC.  The mechanics are, though.  So I rename Twilight into Shadow's Kiss and then set about using the runeblade.  Now I want to attach it to an NPC BBEG in a 5e adventure I am making to sell, say an Orc Chieftan.  That OGC isn't in the 3PP.  To use it I would need to get Orc from the 5.1 SRD that is in CC unless I wanted to trust the OGL 1.0a and use it that way.  Same with all the other monsters I include in the adventure that are from the 5.1 SRD.  Then I create two new monsters and designate those names as PI of mine and the mechanics attached as OGC.  If I want some monsters to use spells, I need to get those from the SRD 5.1 as well.