I'd have to double-check the code for Unique Replacement versions so far as spawning units goes. I may have originally accounted for that in the code that reads from the RASL xml-tables and determines which units to spawn.
I wrote the original code you are running about a week after Civ6 was first released, and parts of the code I have to re-read in order to remember what it was that I was doing in those sections. I originally wrote the code for the RASL mod before there was a Civ6 SDK and before there was a Civ6 FireTuner available to help people along in the process of testing out their mods.
But in looking for the "center-tile" from which to calculate the unit-spawning, the current version of the RASL lua that you have is specifically looking for UNIT_SETTLER and nothing else, and if it does not find a UNIT_SETTLER, it does nothing so far as spawning units. Which is one of several reasons it fails to implement when someone is using Gedemon's TSL mod(s), and is most likely the reason it is failing for the super settler mod.
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I looked through the code again. It is not written to account for Unique Replacement units. If you are stating UNIT_SCOUT in table <RASL_Units> and are instead getting a Unique Replacement UNIT_SOMETHING_ELSE then the game-core must be intercepting the attempt to give a player the wrong type of unit as defined by what is in the <UnitReplaces> table. Such a substitution of a default unit for a player's unique one is not being conducted by the RASL lua code.