No, I think you understand it wrong. They will not use your private channel to route other payments. 
But will make private payments between 2 acinq peers to go straight between them. Is not any limitation, you can still make payments as usual.
You never used a LNbits? It would be the same as sending between 2 LNbits users.
So you will be able to send phoenix to phoenix user even with 0 fees.

I'm still not 100% sure if that's actually it.

Yeah making it more efficient by using your unannounced channel to route a payment. This seems to limit the functionality of a private channel

BlokchainB

When relaying payments, we want to select private channels first and keep as much liquidity available as possible in public channels, to ensure that we don't send a channel_update that would otherwise disable the public channel (and thus make the private channels also unusable since they aren't visible by path-finding algorithms) or limit the htlc_maximum_msat of this public channel (which also indirectly applies to private channels).

https://github.com/ACINQ/eclair/pull/3248

Stacker Saloon

saloon

> *by t-bast*
>
> When relaying payments, we want to select private channels first and keep as much liquidity available as possible in public channels, to ensure that we don't send a channel_update that would otherwise disable the public channel (and thus make the private channels also unusable since they aren't visible by path-finding algorithms) or limit the htlc_maximum_msat of this public channel (which also indirectly applies to private channels).
>
> https://github.com/ACINQ/eclair/pull/3248