Efficient way to create lookup table from multi-value column

I am not sure what your proposed solution is, but your "

Source = Table.FromRows...

is equivalent to my 

(FileUrl) => 
let
    Source = Web.Contents(FileUrl),
    Decompress = Binary.Decompress(Source, Compression.GZip)
in
    Decompress

and both have to appear in two different queries in order to produce two different tables (Movies and MovieGenres). So, I don't see how this solves the problem.

BTW, you don't have to unpivot etc. If you split the multi-valued column to rows directly, the code is simplified (and is more flexible, since your code assumes there are only 2 different values):

The code in that case becomes:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUUrMy8xNLMnMz9NRKM7ILypRitWJVjICSpSnFpekFgGFi/JzE/OSU8ESxkCJ5Pzc1JRKHYXcSpCCSqXYWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", Int64.Type}, {"Column2", type text}}),
    #"Split Column by Delimiter" = Table.ExpandListColumn(Table.TransformColumns(#"Changed Type", {{"Column2", Splitter.SplitTextByDelimiter(",", QuoteStyle.Csv), let itemType = (type nullable text) meta [Serialized.Text = true] in type {itemType}}}), "Column2"),
    #"Changed Type1" = Table.TransformColumnTypes(#"Split Column by Delimiter",{{"Column2", type text}})
in
    #"Changed Type1"