Well it sounds like they replaced their native habitat with a phosphate mine. Seems like a wholesale displacement. Hard to imagine surviving not just your population hit but your entire ecosystem
You’re either severely overestimating the size of the mines or underestimating the size of Christmas Island. On satellite images[0]you can see where there has been phosphate mining, but it’s also very clear that it hasn’t replaced the habitat of the shrew.
[0] https://maps.app.goo.gl/3e55Y1Q3AvMSuccz6?g_st=ic