Help! 2005 Prius alarm during jump start

Hi friendly people!

We have a 2005 Prius, a 2008 Prius, and a 4 month-old baby. Due to the baby, we haven't really done much driving in the two cars, and the 2005 Prius was only driven a few times over the past few months. 3 weeks ago, he was driven from the front of the house to the back of the house and has been there since.

Today, I found that he won't start at all. We have a charged 12.7V car battery that I tried to use to jump him. I am confident in the connection polarity, but immediately upon connecting the final negative clamp, the horn starts blaring. The doors are unlocked, the key is in the ignition, and the metal key is in the door lock based on related threads that I had read. Pushing the anti-alarm button or unlock buttons did not appear to work to stop the alarm. I've tried various combinations of taking the key out of the ignition slot, having the physical key in the door lock, unlocking the doors from the inside, etc. So far, nothing I've seen in other help posts has worked, although I have not yet tried installing a new Prius 12V battery.

Note that I have not actually tried to start the car yet while jumping it since all the guides say to let the battery charge for 5 min first and the sensitivity of Prius electrical system makes me nervous so I was being cautious.

Any ideas?

1. How do I get the car jumped without the alarm issue? Should we just try to wait out the alarm for the 5 mins that are recommended to charge the battery before trying to turn the car on?

2. Should I replace the 12V battery? It's about 5 years old, last replaced by a dealer with an official battery. No problems and it had a clean bill of health at the check-up 4 months back. I'm inclined to replace it after this, but not sure.

Thank you very much!