Is the hdSeedid in Bitcoin Core 0.17 RPC the actual seed itself?
The hdseedid is the same as the hdmasterkeyid (the latter is retained for legacy reasons, it was renamed to hdseedid for better clarity). It is the HASH160 of the HD seed.

If you wish the retrieve the seed, unlock your wallet and run:

dumpwallet /some/filepath/wallet.txt


In the dumped wallet, you should find an xprv key, which is your master extended key. Note that Bitcoin Core does not support BIP39, and the xprv cannot be converted to seed words.