The OP claims he did the implementation (so he was the software engineer too?):

> I designed and implemented most of the tools that are keeping it running so I think I’m qualified to talk about it.

I read this as they built the “tools” (automation, orchestration, monitoring, etc.) for this system, not the system itself; which aligns with the common definition of SRE.

SRE is a mix of both. The expectation is you are able to write and understand any code the team is responsible for.

SWEs can share the oncall rotation with SRE.

Yes that is the normal case. The post was refuting the assertion that one engineer can run these services indefinitely as previously the OP had the help of SWEs oncall and also fixing bugs.