A key flaw in this argument is that it is pretty much accepted that the carbonates containing the magnetite were never raised to the high temperatures that the shock heating theory requires:

New Study Boosts Idea of Past Life on Mars.
Office of News and Public Affairs
University of Wisconsin-Madison
March 13, 1997
"The isotopic procedures employed by Valley and his colleagues were developed specifically for the Mars rock. Results contradict claims that the carbonate globules found in the rock were formed at blistering temperatures too hot to support life, or were formed on Earth, two primary arguments advanced against the meteorite as evidence of past life on Mars. "Everything we see is consistent with biological activity, but I still wouldn't rule out low-temperature inorganic processes as an alternative explanation" said Valley. "We have not proven that this represents life on Mars, but we have disproven the high-temperature hypothesis."
"Valley said the high-temperature origin hypothesis relies on a set of thermodynamic assumptions that don't measure up on Earth, and therefore don't apply to an ancient Mars that may have had conditions more conducive to life."
http://www.jpl.nasa.gov/snc/news3.html

Low-temperature Origin of Carbonates Consistent with Life in ALH84001
Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology, SOEST, University of Hawai'i
"Detailed analysis of their magnetic data lead Kirschvink and his colleagues to conclude that the magnetic grains were not heated above 110 oC, within the range in which life can exist. They suspect that further measurements may suggest a temperature as low as 40 oC."
"Although they cannot estimate the temperature precisely because of the lack of equilibrium, the similarity to carbonates formed at low temperature (certainly less than 300 oC ) on Earth leads Valley and his associates to conclude that the carbonates in martian meteorite ALH84001 formed at relatively low temperatures, too. Clearly, more work is needed to pin down the temperature more precisely-300 oC is too high for life to have existed."
http://www.psrd.hawaii.edu/May97/LowTempCarb.html

Paleomagnetic Evidence of a Low-Temperature Origin of Carbonate in the Martian Meteorite ALH84001
Science, Vol. 275, No. 5306, 14 Mar 1997, pp. 1629-1633.
http://www.sciencemag.org/cgi/content/abstract/275/5306/1629 [full text]

Low-Temperature Carbonate Concretions in the Martian Meteorite ALH84001: Evidence from Stable Isotopes and Mineralogy
Science, Vol.275, No. 5306, 14 Mar 1997, pp. 1633-1638.
http://www.sciencemag.org/cgi/content/full/275/5306/1633 [full text]