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.