How to tell a girl I'm talking to someone else without telling

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Matched with a handful of women on Tinder a few weeks ago.
First of the mentioned two is this 8/10 cutie that I knew from a few parties of mutual friends. I briefly spoke with her on Tinder, before asking her if she was busy that Saturday night.
She never got back to me, so by a few days later, and seeing her briefly around campus, I assumed that she just wasn't interested. Not a big deal. So I moved onto the next one. Really hit it off with this one girl from my hometown that is coming to my university in the fall. We're really getting along well, and she's already been flirty with me. Honestly could see something with this girl if all goes well. Smart, good humored, good head on her shoulders, shows interest and isn't afraid to tell you.

The 8/10 messaged me back after a week and a half, also on a Saturday, saying "lol I just saw this, but not today, I don't have anything."
How do I respond in a way that conveys "Sorry, I figured you weren't up to it, so I started talking to someone else and hit it off with them." without completely closing the door to the opportunity, say if in a month or so it doesn't work out with this current girl?
>Why are you worried about another girl if you like the current one?
Because I've been through the loop a few times of seeing a girl that seemed genuinely interested and then things didn't work out after a month. I'm trying to keep my bases covered.