Learn to walk before you try to run. Posters are still worth something on your application and you can use the data from the poster to write a publication later. They're also great practice for getting the feel for research in a low-stakes environment. This sounds like an easy W for your application and I suggest taking the PI up on it.
Very possible they're testing you or getting a feel for you, first. It's rare for a PI to start you directly off with a manuscript, especially if you do not have a masters, lots of experience publishing/doing analysis, or already know the PI very well.
It's also uncommon for PIs to offer a realistic chance at being immediately productive, so take it and run with it. Best case, this PI will see you have potential and let you augment this data into a paper. Worst case, you get the poster and can tell the next PI that you have research experience in the field and are eager to do more.
I'm about to present my 10th poster and publish my 3rd and 4th manuscripts. Let me tell you, I wouldn't have been able to write a manuscript competently without doing some grunt work on posters first. Learn to walk.