In that context, wouldn't the Comintern have wanted other countries fighting the Germans as soon as possible?

Quite the contrary actually. The Comintern ordered communist parties around the world to condemn the war as an imperialist struggle. While I do not have information directly on the situation in the United States, I can comment on this matter for the situation in the United Kingdom. (For my MA thesis I looked at the British volunteers of the International Brigades. In that context I discussed the political situation in Britain pre WW2.)

The leadership Communist Party of Great Britain (CPGB) initially welcomed the declaration of war on Germany by the British government. The struggle against fascism played a major role in the rhetoric of the party and in the 1930's the British streets had been the scene of many streetfights between members of Oswald Mosely's British Union of Fascists and members from various anti-fascist groups such as the CPGB. The Battle of Cable Street was the most notable of these. When the Spanish Civil War broke out many regarded it as the next phase in the struggle against fascism. The volunteers of the International Brigades eventually returned home roughly six months before the end of the war. Many of them, such as Tom Wintringham, argued that a war with fascism was inevitable and that the UK would do well to prepare itself for this.

As noted, when the war did break out in September 1939 the CPGB welcomed it as part of this struggle against fascism. Now, a few weeks later the CPGB leadership reversed its position after receiving instructions from the Comintern to do so. Harry Pollitt, secretary of the CPGB, stepped down over this change of course, but eventually recanted. He explained his initial reaction as as a consequence of the personal responsibility he had felt to the sacrifices that the British volunteers of the International Brigades had made in the Spanish Civil War. After the invasion of the Soviet Union by Germany in 1941 Pollitt returned as secretary of the CPGB. A position he would continue to hold until 1956.

Sources:

• Buchanan, T., 'Holding the Line: The Political Strategy of the International Brigades Association', Labour History Review 66, 3 (2001), 294-312.
• Buchanan, T., The Impact of the Spanish Civil War on Britain: War, Loss and Memory (Eastbourne 2007).

(To the staff of /r/AskHistorians: This is my first in-depth post at this sub after lurking for quite some time. If it strays to far from the original question, please let me know.)