It looks to be official ones, at the very least. Enough for her to do a free talk stream celebrating it.Isn't this one of those self-produce goods that some liver occasionally had (something like Yang Nari's a while ago), I don't know the procedure behind this on niji's management but also one of the reasons they put these types of goods on Booth instead of the usual niji site is that because they're being handled directly by pixiv factory who then manufactured the goods itself, all you need is an image you'd like to slap to your goods.
Pretty surprising though, hope the rest of ex-id follow through with the idea
Siska is one of the more known livers within ex-id though, she's a moba/fps-focused streamer although she doesn't orbit around njen as much as mika does
Yes, she's well-known but still somewhat under the radar. She still gets some attentions outside of ID but generally she still relatively obscure to casual nijifags unless you into ID specifically.