I took a look to see if I could reproduce the issue and figure out why iOS isn’t working and I honestly have no idea. I tried multiple iOS devices on BrowserStack and I have a test device that is an XS max on OS 13.9 and the video played for me in all these places. I then sent it to my friend who has an iPhone 12 on OS 14 and the video did not work. We then tried her partner’s iPhone and the video worked just fine.
My best guess is its either a network issues, a device issue or possibly something with how the video is actually being loaded due to the network or device. I did find this information from apple about how to properly deliver video on safari: Delivering Video Content for Safari | Apple Developer Documentation.
Since this theme is designed and supported by Shopify, maybe there is a chance they could help directly? However since this question has been around for quite awhile I wouldn’t hold my breath. Out of my own curiosity I might try to figure it out a different day.
Sorry I don’t have a solution for you!