So, what are these 'sun bird ships' or whatever and why can't they be beaten?

When one of these seemingly indestructible ships ventured to the land of those other elves on that other continent they got absolutely hammered, so why can't they be destroyed in combat?

Not sure where you got "indestructible" from.

As for what they are? Theories. Not even MK seems to have a solid idea as he contradicts himself. So I take his contradictions and combine them into one concept.

They glide from the sun[Anu] in times of great need. Which is to say, they serve only in Alinor's defense. They glide from the Sun in unison, shooting sun beams out of their beaks or whatever. Which gives the Aldmeri Navy some aerial support.

Just get the Dragonborn to shout Storm Call.

MK really svcks at making sense and linking lore together. The sun is a hole yet according to him Sunbirds are giant birds made out of the sun...what? How can something be made out a hole and if he means magic from Aetherius then why not say so?
TES lore is both interesting and infuriating because MK is a madman and craps out headcanon then tries to rewrite lore or work it into a loophole to make it official canon...like how Miraak is now the first Dragonborn because he was the first to eat their soul, trixxy hobbitses.

He did have to making several compromises tto his own lore due to gameplay reasons.

*COUGH* Cyrodill was a diverse jungle *COUGH*