Not with the placard. An outsider couldn't get inside. That is what I understood I mean. An outsider couldn't enter the place unless someone killed Mac first.
We don't know what the placard does, specifically, other than protect a place. We don't know if it would do anything other than reinforce the threshold, or if it would do something to stop mortals with ill intent as well.
If Justine had walked in there, it might have let her in, if she did so under her own power and without ill intent.
It was first mentioned in Death Masks.
I stand corrected.
And if it is neutral territory, everyone is welcome, including the Fomor, not to tear up the other patrons, but welcome because someone neutral doesn't take sides... Thus by smearing his blood on the Placard and hanging it up, Mac took a side against the Fomor.. They weren't getting in his place.
The placard was set to defend the place and make Mac the conduit or link to the place. We don't know anything about how it works, but if someone showed up looking for shelter, including the Fomor, it might have let them in. We don't know the rules of how it works.
So saying he took sides to me is an overstep. He just did more of what he was already going to do, but at severely higher risk to himself.
It was all about the Placard and the meaning of it that Mac was reacting to. Not Harry figuring out what he is, Mac wasn't upset,...
I don't know why you're ignoring the text. Here's the direct quote.
“You know what it is,” I said.
Mac rocked back half a pace. He looked from the sign to me. He didn’t quite lick his lips in nervous guilt, but it was pretty clear that he didn’t like that I’d realized what he knew.
The text says it's Harry's knowledge that upsets him, not the placard.