Its not alliance. Its an attempt to create "modern muslims".
And islam has a lot in common with socialism. Blasphemy laws are present in all socialist countries.
While muslims will take full advantage of free speech to argue how Muhammad's marriage to Aisha was so cool, the moment they get power anywhere, they do crackdown on free speech.
Same with socialists. While socialists will claim to be oppressed when someone bans them from arguing how Stalin was a great person, the moment Socialists gain power is when there is crackdown on free speech.
And what unites people better than common enemy?
You see, muslims and socialists both have a common enemy: Capitalist Christian conservatives.
Muslims obviously want to overthrow Christianity as dominant religion, and Socialists likewise dont quite like US capitalists who just happen to be Christian.
So for muslims, building relations with Socialists means dividing USA further and weakening it.
For socialists, getting support of muslims means eventually outnumbering local conservative population.
Because Socialists and leftists have low birth rates, they ally with muslims to improve their numbers.
And because muslims need support of some local population to enter country better, they ally with leftists.
But if it wasnt for Christian conservatives, leftists and muslims would be enemies. their goals essentially contradict because leftists are more often atheists and seek abolishing religion, where muslims seek making religion mandatory.
Economically, they are similar. Islam is basically mild socialism, with extended social programs but managed religiously instead of by government.