One honestly wonders how they came up with the ~1% or ~0.1% of the affected users?
How do they differentiate addresses/seeds created through their wallet from those create through other wallets?
Because they log everything you do. It's a closed source wallet which communicates exclusively through their servers. They know exactly how many users they have, and exactly which addresses belong to whom.
What is the best choice for me? and Why?
Sell all the shitcoins for BTC and then store the BTC in either an open source hardware wallet such as Passport or on an airgapped cold storage device using Electrum.
If you want to keep holding shitcoins for some reason, then you are going to be stuck using insecure or closed source wallets. Your best bet will be some multi-coin hardware wallet.