Coming back to this, since I wanted to be sure, I asked Solactive themselves. Here is the reply I got from Daniel Wendelberger, analyst for Solactive:

Hi -------,

I hope you are well! The selection of relevant companies is done by ARK for the Total 3D-printing Index. As you said, we are only calculating, publishing, and distributing that index.

Let me know in case you need anything else!

Best regards,

Daniel

So my interpretation was correct: Ark themselves decided to put Pyrogenesis into the index, not Solactive. Which is great of course, Ark wanted to buy Pyro and made the concious decision to include them.

Regards