The usage of one is grammatically correct in this context, but it is a mistranslation of Robespierre's speech. Here is the original text:
Déjà, pour éterniser la discorde et pour se rendre maîtres des délibérations, on a imaginé de distinguer l’Assemblée en majorité et en minorité, nouveau moyen d’outrager et de réduire au silence ceux qu’on désigne sous cette dernière dénomination.
The word on can be translated in English as one (any person) or as somebody (a specific but unidentified person). One is used to describe the normal behaviour of any typical person: it simply doesn't work for the first usage of on
a typical person formulated an evil plan
it has to be
a specific but unidentified person formulated an evil plan
Robespierre is therefore suggesting in the second clause that somebody formulated an evil plan, and in the final clause that the same particular person would decide who was in the minority: an appropriate pronoun to use for the second clause would therefore be they. The evil plan only works if they get to decide who is in the minority. Here is my (literal) translation:
Already, to perpetuate discord and to render themselves masters of the deliberations, somebody conceived a plan to divide the Assembly into majority and minority, a new way to insult and reduce to silence those that they designate by the latter term.
Note also that designate is in the present tense, meaning that the designation is not already decided: they can decide now or in the future who is in the minority. This article states that Robespierre was himself "put into the minority" on the 27th July 1794 and was reduced to silence by guillotine the next day.
As you can see, the Jacobin article paraphrases (badly) the first part of the sentence. For the part that is the subject of the question- those whom one designated by this term - the structure of the translation is accurate, but the translation of the word on to one is not correct: the first usage of on makes it clear that the translation should have been somebody, or better they.
who were designated by this term might seem clearer, but it moves even further from the original text.
