Does that mean to make use of fetching an http/s resource requires particle devices to be connected to internet?

My Particle photon is in a closed network does not connect to the internet, but looking at this thread it looks like I have to be connected to internet in order to access the build webhooks for integration.

All I need to access was a http resource inside our closed network from particle photon.

You can use TCPClient or HttpClient to directly talk to your HTTP resource and won’t need webhooks.