Pretty much anywhere outside the USA, the communication of tech workers unionizing is pretty much absent and expecially news about it. This is a big deal, but it doesn't say much about the actual penetration of unions in a given sector. It's a complex topic, but I explain it with the fact that the topic is pretty much uninteresting, unless it's a well-known brand is unionizing. Since most famous tech companies are American, there's enough mass of news there to actually push media outlets to cover news.

In Italy, where there are very few "well-known" IT companies, the topic is completely absent, to the point where IT union organizers from a city don't know about big wins by other IT unions organizers from another city. Nonetheless the narrative is not the thing, and there can be big impacts that become visible to the general public only after sociological studies.

So, long-story short, the fact you never heard about SITT doesn't say much about its effectiveness, just about their ability to communicate.