There seems to have been at least two Svend Aage Munck's who immigrated to USA early 1900s, and the one who immigrated 1921 appears to be the wrong man, as he was born ca 1900 and listed that he was in transit to Mexico.
On 23 June 1930 a Svend Aage Munck age 24 arrived New York who listed forester as occupation and his birthplace and last residence as Copenhagen. Nearest relative was his father Frederik Munck of Lakesgade 23, Copenhagen, and destination was a friend W. C. Walters in Schenectady, New York. He also listed that he had been in US previously 1923-1929.
On 13 January 1939 he apparently returned from a trip to Copenhagen, then listed as a U.S citizen, address Schenectady.
The 1930 listing of his father as Frederik Munck of Lakesgade 23 matches the 1912 address of Frederik Munck in the police register that Paul found, and only conflict is the 3 March 1903 birthdate, versus 9 March 1906 in the U.S. record. Perhaps it was a mistake in the register, or maybe Svend changed it upon immigration.
I don't find him on a 1923 passenger list, but if he was a crew member as the family thought, he would not be listed.