I remember reading this paper when it was published. For those who have read it, one will note that in the Acknowledgments section Janhunen specifically
thanked a "referee for the encourgement to explore the magnetic
anomaly topic further." Whether Janhunen's acknowledgment
reflects the possibility that he intended to develop fully the
electromagnetic portion of his presentation or not is anyone's guess,
though I suspect the latter is more likely owing to the passage "We
do not attempt to discuss in detail the mechanisms that could
primarily generate the 40 kA current in the ocean." Frankly, even
Janhunen notes that the "~ +/- 40 nT magnetic anomalies" over the
northern plains, which were observed in the MGS MAG/ER data by Acuna
et al., can be explained by the (perhaps simpler) model of Acuna et
al. that invokes remanent magnetism.

For what it's worth, though, the paper (very brief for a typical JGR-
Planets submittal), is only offered as speculative piece of work.
Even the author notes that, other than the putative the "anomalous"
magnetic readings, there is a dearth of direct observational data to
support his model. It appears that Janhunen is taking an admittedly
speculative model (i.e., extrapolation of the Galileans analog to
Mars) and attempting to tie in "anomalous" magnetic readings,
thermodynamic modelling, etc. In fairness, though, it's not as if
speculation has not passed peer-review, is it?

Frankly, I believe that Janhunen is, to put it charitably, "reaching" with this paper. The MGS MOC NA imagery (Malin and Edgett)
and the MGS MOLA data (e.g., Frey et al, Head et al., etc.) indicate
that while the "northern plains" are fairly flat and homogeneous at
macroscales, at decameter scales the surface appears rougher and less
smooth (i.e., a "thin" cover where the rims of buried craters poke
through). One might also note that the References section of
Janhunen's paper, with the exception of the citation of last year's
Clifford and Parker paper in Icarus, includes mostly works from 1999
and before. Whether this was due to a lengthy passage of time
between submittal of the initial manuscript and acceptance of the
final version is unclear, but close observers will note that no less
than Jim Head, among others, is backpedaling somewhat from his
original "shoreline" interpretation of Parker's Contact 2, which he
published in Science (and which was cited by Janhunen).