I think what they mean about the seeds is that old user with old seed will not be converted or transfer to new seeds that include both segwit and your old bitcoin addresses. So if you are planning to export the seed and import it to another wallet(like electrum) you can only recover the old addresses without segwit addresses.
That makes sense.
I've only exported seeds to get my hands on forks, and Segwit seems to be problematic for that and forks are dead anyway so it makes no difference to me. Either I didn't read it properly or they could do with explaining it better. Someone might get a shock if they expect their Segwit coins to show up again.
