This is exactly why the bip39 tool is there and my advice to not do it on the webpage itself but rather to "save it as..."
The seed generated by the nano s shouldn't be entered anywhere else than the nano s itself.
That's the only way to be sure that the seed won't get compromised.
Why risk compromising the seed if you can check it using electrum where the private keys won't leave the nano s at all..
This is a known method for recovering lost or "unreachable" coins.
If the nano s contains the private key to this address, they are not lost or 'unreachable'.
And to check that (given that OP didn't verify the receiving address on the nano s), connecting the HW with electrum is definitely a better way than risking to compromise the seed.