No, it’s not true. You wouldn’t ever use magnesium for that anyways, you’d use an actual demolition charge if you were dumb enough to rig every tall building with explosives. But no, nobody is that dumb - the kernel of truth at the heart of that one is that some large buildings have demolition plans drawn up when they’re constructed (important note: the WTC didn’t have that, it’s both very rare to do and a much more modern practice than the WTC buildings), which are just a reference for if/when the building is demo’d in the future. They’re not rigged to blow, though. Hell, aside from all the obvious reasons you wouldn’t do that, chemically most explosives would become uselessly unpredictable after being strapped onto a building for that long. Just an absurd idea.