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Your integrated App service should be able to reach to resources in OnPREM even if they are connected via S2S.
I believe you were referring to : https://video2.skills-academy.com/en-us/azure/app-service/overview-vnet-integration#how-regional-virtual-network-integration-works
In the same document, under "Access on-premises resources" section, you can see,
No extra configuration is required for the virtual network integration feature to reach through your virtual network to on-premises resources. You simply need to connect your virtual network to on-premises resources by using ExpressRoute or a site-to-site VPN.
Hope this helps.
Thanks,
Kapil
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