He may not have been around for it. Logically, if the Veil did destroy the ancient elves and sever their special connection to the Fade, then it's likely that those affected don't remember what happened. So even if Abelas was present to this alleged event, that doesn't mean he would remember it properly.
Seeing the "impossible" made possible isn't automatic proof of godhood, especially in a world where magic exists.
I'm fairly certain he does remember:
This veilfire script was hidden in the Arbor Wilds. It's so old it cannot be translated into any known language.
There are whispers fom the Well of Sorrows. It's impossible to understand the entire text, but certain parts suddenly reveal a shadow of their original meaning.
"We are trapped. The ones born here do not understand the keenness of what we have lost, or why so many of their elders weep as they enter uthenera. The new ones are faithful to Mythal, but do not understand what she was in her fullness. Without the wise to lead them, they will lose what they should have been.
I will teach them. They must serve. We must prepare for those who cast Mythal down. I shed my name the day I began her service. I shed my new one again, now that she rests. I will only be known by the sorrow that cuts my heart."
For a moment, there is a feeling of wrenching loss. Then it fades.
And the memories for the Vir Dirthara seem to indicate that people were aware of the Veil, even if they didn't understand what it was.
And it may not be proof of godhood to you, but you are not an Elvhen. Besides what you consider a god and what the Ancient Elvhen considered one might differ, the term godhood is up to individual interpretation. Seeing someone transform into a dragon, or survive apparent death, or teleport across the continent might have been enough for the Elvhen.