They mention end-to-end efficiency of 40-45%...

That’s just for heat to thermal and quite optimistic not end to end. “Conversion back into electricity is 40%-45%”

More realistic end to end numbers are likely in the 30% range which means summer electricity needs to be vastly less valuable than winter energy before you nominally break even and start repaying the investment. Further you instantly lose all the electricity required to heat the mound up to working temperatures. IE: If you can only operate between 550C and 650C then going from 20C to 550C needs to happen before you can extract any energy and you don’t get that investment back. On the other hand if you’re a chemical plant that needs 200C things start looking a lot better.