First the Declaration says he arrived on or about March 17, 1873 .... after all 33 years is a long time to recall the exact date of arrival... so he may not have arrived on that exact day or... year.
Yes there were occasional stowaways but they would have been detected.
But a more common thing that could have happened...
1) he used someone elses passport
2) or his name was improperly transcribed by modern day transcribers and they misinterpreted his name on the manifest.
There were much, much fewer stowaways than stories about stowaways :)
Did you notice there is a Franz SIKORSKI aboard the Mosel arriving 1874.
Also by now you know that he was Polish from that part of Poland occupied by Prussia or Germany at the time of his birth
Sikorski Name Meaning Polish and Jewish (from Poland): habitational name for someone from any of several places called Sikora, Sikory, or Sikorzyce, named with sikora ‘titmouse’.
Source: Dictionary of American Family Names ©2013, Oxford University Press
Also the correct spelling of his given name would be Anton or Antoni
You really ought to consider posting to the Poland board