I think you do not understand. It is optional to connect to your OWN Electrum server, not that the whole electrum connection is optional.

Phoenix cannot work without being connected to an Electrum server. And that Electrum server work only with a full core node.

Is Phoenix a LN node? YES (with limitations). Is Phoenix a full bitcoin node ? NO

No

No, it does not connect to one "simply," it connects to one "complicatedly."

Phoenix runs a lightning full node in your phone but not a bitcoin full node. Lightning (the software) and bitcoin (the software) are different and Phoenix only has a full copy of the former.

For the latter, it connects to an electrum server (which is a full node) but does not "simply" ask it for data. It asks it for data and then uses an incomplete copy of bitcoin (the software) to check the validity of some of it.

Specifically, it uses a stripped down and customized copy of bitcoin (the software) in "SPV mode" (simplified payment verification) to check if a transaction you received was (1) included in a block (by validating a "merkle proof of inclusion") and (2) if that block was expensive to produce (by validating the block's "proof of work").

If the incoming transaction passes those two tests, Phoenix considers it legit, on the grounds that it is unlikely that someone would spend a large amount of money to produce a proof of work for a fake block in order to trick a phone into accepting a fake transaction. Remember that phone wallets are only designed for small amounts anyway. For large amounts, use a full node.

ACINQ pioneered the lightning service provider (LSP) model. When you install Phoenix Wallet you are installing a lightning node on your phone. It is not a full bitcoin node. It's not even a full lightning node. Instead, it delegates all of the heavy lifting to the ACINQ service. Your node opens one channel to the ACINQ node and relies on ACINQ to handle lightning routing, liquidity, splicing, and a bunch of other stuff that requires an always-online service. In return, you pay fees to cover the cost of these services.

ACINQ deployed the first mobile lightning wallet and have been at the leading edge of mobile development for ~7 years now, by my count. They are the unsung heroes of lightning, IMO. I'm a huge Phoenix stan.

They usually blog their biggest engineering achievements. They don't post often but when they do it's a banger. Highly recommended reading! https://acinq.co/blog

Is Phoenix a LN node? YES (with limitations). Is Phoenix a full bitcoin node ? NO The short answer to your long post.😜