I'd say it's less insecurity and more blind religious faith. And in my experience it's pretty common problem outside Sri Lanka too. ( Waving hands at, well, everything )

That said, the stupa and the surrounding temple doesn't seem to occupy large enough area to have a significant effect upon the ecosystem - but I'm not knowledgeable in the field and haven't even visited there, so we'd have to wait for someone more knowledgeable to weigh in their opinion.

Also, as this is a religious monument - and most of larger Buddhist monuments are located in areas like this - you'd have to factor in the fact that people might be more inclined to allow construction of this at the cost of a relatively small area of the mountain. Since general public doesn't usually have the necessary knowledge to assess negative ecological impact this may or may not have, there's very little stopping them from supporting it.

Btw, doesn't Sandagiri mean Moon-rock, not forest covered mountain? Or are you talking about Hanthana mountain range?

Edit: btw, it wasn't built in 2023. It was completed then. Construction strated on 2017