A different network has nothing to do with it. It is more like the seed phase compatibility, there are no such things as "Bitcoin" seed phrases. Generally, BIP 39 is mainly used since it is universally accepted across many wallets.
Actually BIP means Bitcoin Improvement Proposition, so yes BIP 39 means Bitcoin Seed Phrase. My question is therefore if it was also used by Etherum and it seems like this is the case.
BIP 39 is the most common standard used by the majority of cryptocurrency wallets, yes it is initially being developed and applied within the bitcoin ecosystem, but even the BIP itself doesn't mention BIP39 as a bitcoin seed phrase as a whole. So I don't think we can refer to BIP39 as a Bitcoin Seed Phrase.
And yes, it is. Assuming the wallet is compatible, the BIP39 Seed phrase used for Ethereum, can also be used for Bitcoin.