I think that you'll need to do something more with $LastImport to get the SQL engine to treat the value as a date and not an integer value.
According to FileMaker's SQL reference document, you'd need to do something such as:
'" & Year ( $LastImport ) & "-" & Right ( "0" & Month ( $LastImport ) ; 2 ) & "-" & Right ( "0" & Year ( $LastImport ) ) & "'
(Note that there are single quotes to the left of the first " and to the right of the last " in this expression.
I think that there may be other ways to do this too, but just referencing a variable set to the value of a date field isn't one of them.
This may be one of those other methods? TO_DATE statement
"SELECT * FROM xyz_LEVEL_0_DATA_V WHERE xyz_LEVEL_0_DATA_V.LAST_UPDATE_DATE > TO_DATE(" & $$LastImport & ", 'MM/DD/YYYY')"
Works. You are right - just referencing a variable not working at all.