I also think the Libre readings are what your BM will show in 30 minutes or so.
There are loads of threads about Libre (the Search function on the top right of the page will help you find them).
The lag has been mentioned multiple times and the way it is handled is different for Libre 2 compared to the earlier version.
As you say, CGMs and Flash GMs such as Libre read interstitial fluid which is different to blood. There is a lag between these readings ... it 's only 10 to 20 minutes. However, the algorithm Libre 2 uses to convert the ISR to BG takes the delay into consideration by extrapolating the graph.
I highly recommend reading through some of the Libre threads to understand its limitations, It is a fantastic tool but only if you understand what it cannot and does not do. Blindly using it to replace finger pricks will leave you disappointed and wasting money.