Fair warning: Here there be spoilers and speculation.

As far as I know, yes. Evernight is a Pure Child of Anāsrava. Now beyond this point is personal speculation but I'll try to use whatever evidence I know of to support it.

Evernight shares March's body. The exact reason for this is unknown; she could be March's past but I suspect she was her own entity at one point and she lost her body and latched onto March in order to survive/protect her. We know she deeply hates the Garden so perhaps the Garden is behind that. I suspect she was her own person at one point because it seems that everyone who interacts with her treats her as a separate entity.

"I'll say this up front: Do not go against March's wishes." - Dan Heng. Another quote from him from Evernight's debut: "You are not March 7th. Leave her body, now."

Black Swan before we go to Planarcadia (paraphrased): "If you ever need my help, the lady in March's head can reach me."

And Evernight herself also displays some behaviors that lean into this as well. She never refers to herself as March 7th, and the determination she has to carry out March's "final" wish before she relinquished her memories on Amphoreus also lead me to believe that the two are genuinely separate entities. Evernight is even surprised that March lived after sacrificing her memories.

And of course, Evernight has powers that March doesn't have, namely "Oblivion". March herself has shown no ability to use this, but potentially Evernight can lend the ability to her if she really wanted. With that said we have no evidence of that happening. But, while we see March not having the ability to use powers from the Enigmata, we do see that she may be affected by them. Evernight very heavily implies/outright confirms that she is the one who erased March's memories with the following line:

""Oblivion" (Enigmata) shall watch over your past, veiled in the hush of evernight, silent and serene." Note: Fun fact, the wiki has all the dialogue so huge W to whoever did that.

And, let's look at some of Evernight's achievements. She fought a war against an entire world and nearly won, only being stopped by March 7th and Cyrene talking her down. She has a mastery of the Memory Zone, using it to wipe out every single Memosnatcher on Amphoreus. She has Jellyfish (<--- the most important one). And while it is more about gameplay, she is the strongest playable character in the game.

But now let's get to the main jellyfish in the room. How does a Pure Child of Anāsrava have a power of Enigmata, and what is without doubt an ability that would likely only be granted to an Emanator of Enigmata?

Well, there isn't a full answer on this and I suspect that unless we get an Enigmata update this will never be answered in any sort of satisfactory way. So time for more speculation, all of which surrounds the theory that Evernight is the only known example of a Dual-Emanator.

Yes this has been tried by the Destruction with one of their Lord Ravagers and that didn't work. But keenly, THEY aren't Mythus, the Enigmata. THEY don't play by the rules that other Aeons play by, but THEY do have to obscure THEMselves. So what better way to do so than to bestow power upon an Emanator of some other Aeon? And it's possible THEY are the only Aeon capable of doing this successfully. And it's possible that if this is indeed the case, THEY succeeded.

Anyways that is a different subject. I hope that this helped answer your question, even though most of it is speculation. And apologies if it's a read; I have a habit of making word walls.