Suppose your junta has toppled the government and assumed absolute power. How do you express that in legal terms? The first Institutional Act, unilaterally decreed by the Supreme Command of the Revolution in April 9th, is an useful reference, expressing absolute power in clear terms on its preamble:

What has taken place and continues in this moment, not only in the spirit and behavior of the armed classes, but also in national public opinion, is an authentic revolution.
A revolution distinguishes itself from other armed movements in that it expresses not the interest or desire of a group, but the interest and desire of the Nation.

The victorious revolution assumes the exercise of Constituent Power, which can manifest through popular election or revolution. This is the most expressive and radical form of Constituent Power. Thus, the victorious revolution, as Constituent Power, is by itself legitimate. It topples the previous government and has the power to establish the new. Within it is the normative force inherent to the Constituent Power. It changes judicial norms without being limited by the normativity that preceded its victory. The Chiefs of the victorious revolution (...) represent the People and in their name exercise the Constituent Power, of which the People are the only holder. (...) The victorious revolution needs to institutionalize itself and hurries towards institutionalization to limit the full powers that it effectively has.

(...) Constitutional processes failed to topple the government, which deliberately wanted to bolshevize the Country. Toppled by the Revolution, only it (the Revolution) can dictate the norms and processes of the new government's construction and give it the powers or judicial instruments that ensure its exercise of power (...) To demonstrate that we don't intend to radicalize the revolutionary process, we've decided to keep the Constitution (...) we have chosen, likewise, to keep Congress (...)

It is thus clear that the revolution does not seek to legitimize itself through Congress. It is Congress that receives from this Institutional Act, a result of the exercise of Constituent Power, inherent to all revolutions, its legitimacy.

This was decreed after Congress wasn't giving them what they wanted, and formalized the ongoing purge of the old regime's members. The Supreme Command's power was tied into existing legal understanding by equating it to the People forming a Constituent Assembly. Hence, they can place themselves above Congress and the Constitution. It is an exercise of sheer willpower: they wanted power, they took it without regard for the legal process and then decided what's legal or not.