It turns out Martelli has written about this in English (so was relatively easy to find). Although his argument seems a bit convoluted to me, what he seem to be saying is basically that in construing the political enterprise as a fight against a certain group, it's so much easier to define that group ("the other") in terms of ethnicity etc.
Chantal Mouffe’s proposal takes cognizance of the European left’s failure to halt the rise of the extreme right. She wants it to be realistic: rather than rejecting the concept of populism it is better, she says, to turn it against its dangerous users, better to not discourse in general on the right and the left but oppose a ‘left populism’ to a ‘right populism’ … The formulation is simple; but it is also questionable. Why? Because although categories of popular strata exist concretely the people does not exist but has to be constructed politically.
It cannot be constructed by referring to it nominally or by distinguishing it from its supposed opposite (the elite) but by gathering it around the project that emancipates it at the same time as it allows society as a whole to emancipate itself. It is therefore no accident that in the nineteenth and twentieth centuries the parties of popular emancipation did not peg their principal identification to a sociological denomination, popular or worker, but to the project that they intended to promote. They called themselves ‘social democrats’, socialists’, and ‘communists’; the content of their project took precedence over their social determinants.
They had sound reasons for adopting this approach. In the broad social struggle, the accumulation of mobilisable components is nothing without the binding element that makes them into a coherent force rather than a mere numerical aggregate. To achieve this bonding is it enough that the dominated groups have a common enemy? Finance? It cannot be seen. The elite? Its boundaries are fuzzy, either too extensive or too restrictive. What is more, the elite adversary can be the ‘privileged’ functionary against the private-sector wage earner, the stable worker against the precarious wage earner, those who are too poor to pay taxes against those who are not very much less poor but who do pay taxes. The most convenient enemy is in fact those who are closest – in general this enemy is below us and does not resemble ‘us’. The immediate enemy is the ‘other’, especially when we are repeatedly told that this is the age of the clash of civilisations and the defence of threatened identity.
Frankly, a US journalist has probably argued/conceptualized roughly the same issue better:
The US journalist John Judis writes in The Populist Explosion that left-wing populism is “diadic”, whereas right-wing populism is “triadic”. The former opposes “the people” to an “elite”, whereas the latter always adds a third party, typically immigrants, whom the “elite” are accused of favouring. This is far more categorical than Mouffe is willing to be, given her insistence that politics is riven with uncertainty, emotion and conflict, but it does clarify what exactly is at stake. If the political task right now is to construct a “people” from which a new common sense can be built, the question of how that can be done so as to include strangers and newcomers may be the most important one of the next few years.
I'm not sure I buy this is a 100% dichotomy between the left and right populisms, but it is basically the idea that it's easier to blame the internal enemies/elites when they are also painted as a proxy for some foreign forces. I think that in Latin America a number of "left-populists" (of the "pink tide") have managed to pull this type of feat too by blaming the "neoliberal US" as that foreign entity to which the local elites are (supposedly) subservient to.
In the global North, right-wing populists weave a narrative that charges educated and urban elites with embracing globalization policies that sent industrial jobs overseas while allegedly welcoming Third World immigrants in. They put a nationalist spin on opposition to neoliberal globalization, and demand that government instead put “America [or Britain] first.” In Latin America, by contrast, it is leftist movements that most directly challenge globalization. They portray governmental elites who embrace neoliberalism as prioritizing the needs of foreign capital over their own citizens, serving as vendepatrías, people who sell out their own country.
There's still a valid distinction left between left- and right-populism to be made since the left-populists are more redistributive internally, even though both might be protectionist in trade (as a result of painting foreigners as in cahoots with the local elites).