I think that conception equates utilitarianism with rightness, whereas, Mises thought of the very notion of rightness as a mere convention invented for the sake of utilitarian concerns. And so Mises’ utilitarianism isn’t so much an ethic, as it is an understanding of how ethics comes to be.
Hopefully the following quotes will illuminate his position somewhat.
“Utilitarianism, on the other hand, does not deal at all with ultimate ends and judgments of value. It invariably refers only to means.”
“The notion of right and wrong is a human device, a utilitarian precept to make social cooperation under the division of labor possible”
“In this sense we speak of the subjectivism of the general science of human action. It takes the ultimate ends chosen by acting man as data, it is entirely neutral with regard to them, and it refrains from passing any value judgments. The only standard which it applies is whether or not the means chosen are fit for the attainment of the ends aimed at. If Eudaemonism says happiness, if Utilitarianism and economics say utility, we must interpret these terms in a subjectivistic way as that which acting man aims at because it is desirable in his eyes. It is in this formalism that the progress of the modern meaning of Eudaemonism, Hedonism, and Utilitarianism”
Mises doesn’t say "don’t intervene in the market economy, because to do so conflicts with the end of “satisfying the preferences of everyone involved”. That would be him establishing an end as a third party (even if it alludes to the ends of others), but he takes ends as given. He simply says that, given the ends all political parties profess to have (see quote below), intervening in the market economy will not serve those ends.
“However, if we pass in review the programs of all parties–both the cleverly elaborated and publicized programs and those to which the parties really cling when in power–we can easily discover the fallacy of this interpretation. All present-day political parties strive after the earthly well-being and prosperity of their supporters. They promise that they will render economic conditions more satisfactory to their followers. With regard to this issue there is no difference [p. 181] between the Roman Catholic Church and the various Protestant denominations as far as they intervene in political and social questions, between Christianity and the non-Christian religions, between the advocates of economic freedom and the various brands of Marxian materialism, between nationalists and internationalists, between racists and the friends of interracial peace. It is true that many of these parties believe that their own group cannot prosper except at the expense of other groups, and even go so far as to consider the complete annihilation of other groups or their enslavement as the necessary condition of their own group’s prosperity. Yet, extermination or enslavement of others is for them not an ultimate end, but a means for the attainment of what they aim at as an ultimate end: their own group’s flowering. If they were to learn that their own designs are guided by spurious theories and would not bring about the beneficial results expected, they would change their programs.”