Although I can understand this argument, I don't think it's realistically so. Leninism COULD mean that, but in practice, it's used only to exclude Marxist Leninists from the group. When someone says Leninist, they are so often referring to it in opposition to "Stalinists" that the umbrella model can't really work.
Apparently Leninism is a wider term including ML, Trotskyism, and other tendencies: https://en.wikipedia.org/wiki/Leninism
So it wouldn't be wrong call MLs Leninist, or just Marxist if you want to cast an even wider net.
Although I can understand this argument, I don't think it's realistically so. Leninism COULD mean that, but in practice, it's used only to exclude Marxist Leninists from the group. When someone says Leninist, they are so often referring to it in opposition to "Stalinists" that the umbrella model can't really work.