Like Electrum seed phrase, Iancoleman can not be used to generate Electrum wallet private key from its seed phrase because the way the seed phrase is generated on Electrum is not the same as it is generated on BIP39 wallets.
Electrum seed phrases are generated very differently than BIP39 seed phrases, as you say, but this is not why you cannot import them to Ian Coleman's tool. The reason they do not work in Ian Coleman's tool is because the process of turning the seed phrase in to a master private key is (very slightly) different. Ian Coleman's tool doesn't care how the seed phrase was generated, only how it is used. Essentially the only difference is in the salt for PBKDF2 - both concatenate your passphrase (if you are using one) with another word, but BIP39 uses the word "mnemonic" whereas Electrum uses the word "electrum". If you make that very minor change in the code for Ian Coleman, plus an additional change to have to stop checking for the BIP39 checksum (which does not exist in Electrum seed phrases), you will be able to recover your Electrum addresses.
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I would also suggest doing this in this specific case, but do it with a hardware wallet paired with Electrum rather than an Electrum software wallet.