Why isn't Fn::ImportValue allowed in substitutions for DeployTimeSubstitutedFile

[amalik01suncor]

Context:
I have two stacks:
Stack A: Creates the Lambda functions and exports the ARNs of the Lambda functions created as part of this stack.
Stack B: Creates a an API Gateway that uses the DeployTimeSubstitutedFile to replace placeholder values for the lambda functions in a Swagger file. For the substitutions I am passing in a dict where each variableName's value is set to the Fn::ImportValue that imports the ARNs from the Stack A but I get this error:
RuntimeError: Invalid CloudFormation reference. Key must start with any of "Ref" or "Fn::GetAtt" or "Fn::Select". Got {"Fn::ImportValue":"SamLoyaltyPartnerLinksLambdaArn"}

So I looked at the code: https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-s3-deployment/lib/render-data.ts#L63
and it only allows: ['Ref', 'Fn::GetAtt', 'Fn::Select']

So i took the synthesized CFN template generated by the CDK and saw that it had the following configuration for the replacements:

      SourceMarkers:
        - <<marker:0xbaba:0>>:
            Fn::Select:
              - 6
              - Fn::Split:
                  - ":"
                  - Fn::ImportValue: AlphaLambdaArn
          <<marker:0xbaba:1>>:
            Fn::Select:
              - 6
              - Fn::Split:
                  - ":"
                  - Fn::ImportValue: BravoLambdaArn
          <<marker:0xbaba:2>>:
            Fn::Select:
              - 6
              - Fn::Split:
                  - ":"
                  - Fn::ImportValue: CharlieCognitoUserPoolArn

That begs the question if Fn::ImportValue is okay as a nested value why isn't it acceptable at the topmost level.

I replaced that with this in the generated template and it worked fine:

        SourceMarkers:
        - <<marker:0xbaba:0>>:
            Fn::ImportValue: AlphaLambdaArn
          <<marker:0xbaba:1>>:
            Fn::ImportValue:  BravoLambdaArn
          <<marker:0xbaba:2>>:
            Fn::ImportValue: CharlieCognitoUserPoolArn

[amalik01suncor]

The workaround I have right now is to use this hacky solution for building the substitutions map:

substitutions={
   "alpaLambdaARN"=Fn.select(0,[Fn.import_value(AlphaLambdaArn)])
}

which is really not ideal