After following the link to Monticello, I think that the idea you are mistaken is, in fact, a mistake; rather, the adulterated quote attributed to JEfferson is, in fact the POINT of the matter.
I excerpt the full text Monticello uses from source material here:
"A principal source of errors and injustice are false ideas of utility. For example: that legislator has false ideas of utility who considers particular more than general conveniencies, who had rather command the sentiments of mankind than excite them, who dares say to reason, 'Be thou a slave;' who would sacrifice a thousand real advantages to the fear of an imaginary or trifling inconvenience; who would deprive men of the use of fire for fear of their being burnt, and of water for fear of their being drowned; and who knows of no means of preventing evil but by destroying it.
The laws of this nature are those which forbid to wear arms, disarming those only who are not disposed to commit the crime which the laws mean to prevent. Can it be supposed, that those who have the courage to violate the most sacred laws of humanity, and the most important of the code, will respect the less considerable and arbitrary injunctions, the violation of which is so easy, and of so little comparative importance? Does not the execution of this law deprive the subject of that personal liberty, so dear to mankind and to the wise legislator? and does it not subject the innocent to all the disagreeable circumstances that should only fall on the guilty? It certainly makes the situation of the assaulted worse, and of the assailants better, and rather encourages than prevents murder, as it requires less courage to attack unarmed than armed persons."
As a result of analyzing the language, context unknown, I concede - it sounds as if Jefferson was making a note of the argument, rather than casting a judgement.
Meaning: The False Idea - as stated in the FIRST LINE of the source - is that making a law that removes weapons from the "rabble" will only disarm those who would obey the law. The idea of disarmament is, itself, the "false utility."
Bill's convinced me, and, after all, the statement is consistent with Jefferson's attitude towards guns expressed elsewhere. What do you think?