It's a very good point re: sNDA (US) vs Initial MAA (EU)
But that said, it all comes down to content reuse across the two dossiers.
For the sNDA, certain information wouldn't necessarily need to be included, e.g. previously submitted nonclinical studies, manufacturing information, etc, that would indeed need to go into the Initial MAA, as that information obviously wouldn't been filed previously in an EU marketing application.
For sNDA, Amarin would just cross reference their US applications for which that information has previously been submitted, while submitting the relevant new clinical information in the new sNDA.
For Initial MAA, Amarin can reuse submission components created for their new sNDA, as well as those documents being cross referenced within the sNDA from their other previously filed applications.
The key enabler for content reuse is the common submission format for both US and EU submission components. It is harmonized to an ICH standard called Common Technical Document (CTD), or eCTD, which is the electronic presentation of these dossiers. eCTD is a requirement for submissions in both US and EU (and other ICH regions).
I hope this helps and did not put you to sleep :)
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