The instructions suggests that SLIP39's "Master Secret" is equal to the entropy used to generate BIP39 mnemonic.
Thus it's possible to covert and revert.

But the examples in the repository doesn't restore the same set of keys when restored to Electrum's SLIP39 restore option.
Reproducing it with random BIP39 seed produces the same result as well.
Perhaps an issue with Electrum's implementation?

(Saint-loup's original query is related to Electrum BTW)