  Observations of the first interstellar minor object 1I/2017 'Oumuamua did not
reveal direct signs of outgassing that would have been natural if it had
volatile-rich composition. However, a recent measurement by Micheli et al
(2018) of a substantial non-gravitational acceleration affecting the orbit of
this object has been interpreted as resulting from its cometary activity, which
must be rather vigorous. Here we critically re-assess this interpretation by
exploring the implications of measured non-gravitational acceleration for the
'Oumuamua's rotational state. We show that outgassing torques should drive
rapid evolution of 'Oumuamua's spin (on a timescale of a few days), assuming
torque asymmetry typical for the Solar System comets. However, given the highly
elongated shape of the object, its torque asymmetry is likely higher, implying
even faster evolution. This would have resulted in rapid rotational fission of
'Oumuamua during its journey through the Solar System and is clearly
incompatible with the relative stability of its rotational state inferred from
photometric variability. Based on these arguments, as well as the lack of
direct signs of outgassing, we conclude that the classification of 'Oumuamua as
a comet (invoked to explain its claimed anomalous acceleration) is
questionable.
