As someone with 8 years of experience in SRE in Google: I wouldn't be so sure about that. Most outages require only rudimentary understanding of the particular service. Pretty much "have you tried turning it off and on?", with the extra step of figuring out which piece of the stack needs the kick. Hence, there are many SRE teams that onboard lots of services with this kind of half-support. The on call only performs generic investigation and repair attempts. If that doesn't help, they escalate to the relevant dev team, who likely will only respond in office hours.

Only the important services get dedicated oncalls. Most important ones will have both 24/7 SRE and dev oncalls.