That chart is really interesting, thanks for sharing. Here's the thing to remember about arm strength at SS, while it's important, it's not the be all, end all if your can throw accurately and transfer quickly. Take Dansby Swanson from the Braves, he has 19th percentile throwing but was worth 20 OAA and 15 fielding runs.
So even if Holliday's arm does not improve and judging by a few videos I've seen this spring it does not appear it's going to be a strength for him, that does not necessarily mean he can't become a good major league shortstop.
Now is he better than Gunnar Henderson right now? No, I don't think so and besides Law, I don't think any other evaluator would say so. Will he be better in a few years if Gunnar's range, which is average now, becomes below average due to him getting bigger and older? Perhaps.
I think over the next few years at least, Gunnar is going to be at SS and Holliday at 2B. But it would not surprise me in 3-4 years if Gunnar doesn't move over to 3B and depending on who is ready (remember the Orioles have some highly thought of International SSs developing) Holliday could move to SS or he may just stay at 2B because he's comfortable there by then and because he'll probably end up a plus defender at 2B.