Phoenix is already a custodian. I doubt what they have is always going to fly under the radar. Create an on chain address on Phoenix and you'll see that it comes directly from their servers.
There's a good argument that the time scale involved in getting a new channel isn't really a custodial arrangement. That's akin to claiming a money exchange booth is also a custodian because they physically have your money for a minute or two while they count out the foreign currency they're about to give you.
Very different circumstances to something like Wallet of Satoshi, who has had full control of the $5 I've had on my WoS wallet for the last few months.
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