There is no need to deposit coins into FTX, make the swap, withdraw from FTX, and send the coins back to your Ledger. The swap is apparently non-custodial in nature.
Time will tell how true that claim is. If users get their assets frozen for whatever reason, we can no longer talk about self-custody.
I don't agree that you don't lose the custody, neither that you don't send the coins to ftx.
The swap itself happens when you first send your coins to FTX (Authorizing the transaction in your device). At this moment you don't have the custody, even for just a few minutes.
Then FTX will send you back the coins you just exchanged.
I don't think this is risky. But you need to provide all your KYC details to trade more than 2000 usd, and all your ledger live data/history will probably be shared with ftx.
There is very little convenience gain, very high privacy risk and also you are getting a worse exchange rate compared to using ftx itself.