However, lots of interesting plasma physics effect occur between us and the FRB. The dominant thing that happens is that low frequencies arrive later than high frequencies (as they travel slow in a plasma). This causes the FRB to be smeared over many seconds rather than a millisecond. There's also multi path interference effects and an whole bunch of other stuff that happens. So actual the temporal structure of the burst tells you much more about the intervening medium than it does about the source.