Clearly Shopify has at least one single point of failure in their architecture.
Also sounds like they are not feature flagging their code changes well (to enable progressive per-tenant enablement) and they aren’t implementing redundancy of services with rolling code change approach and non-breaking changes.
Yes, it is very hard to get this consistently right, but at least you can mitigate and avoid having to be so draconian with code freezes.
I suspect they have most of this, they are a pretty tight engineering organization. no one is perfect however, especially at the speed they innovate.