Since the 2018 world cup they've kept a league table of who is best at penalties from every single practice and match the England squad plays. We know that for certain. It's been reported they based last nights list of 5 on that league table.
I expect what happened is Saka got picked because he was high up the league table among the squad. You can see in the video going round, Southgate is dithering on the last taker, probably because he didn't want to pick such a young kid, but in the end as is very typical for this England side, they trusted the numbers.
5th is the best place to put him also. A lot of shootouts don't get to the 10th penalty.