Why does VS Code not know what properties my SurveyQuestion type has?

README.md

config.d.ts

// in projectroot/types/
interface MapConfig {
    applicationName: string;
    schoolName: string;
    introText : {
        title: string;
        body: string;
    };
    redirectURL: string;
    mapbox: {
      /** @see https://www.mapbox.com/studio/ to create style */
      styleUrl: string;
      /** @see https://www.mapbox.com/mapbox.js/api/v2.4.0/api-access-tokens/ */
      accessToken: string;
    };
    /** @see https://www.mapbox.com/mapbox.js/api/v2.4.0/l-latlngbounds/ */
    bounds: MapConfigBounds;
    types : Array<MapConfigFeature>;
}

interface MapConfigBounds {
  southWest : ({ lat: number; lng: number });
  northEast : ({ lat: number; lng: number });  
}

interface MapConfigFeature {
  /** "bike path", "park" etc. */
  name : string;
  type : 'path' | 'point';
  /** hex color */
  lineColor: string;
  description: string;
  /** Name of icon.png file */
  icon: string;
}

survey-question.d.ts

// in projectroot/types/

interface SurveyQuestion {
  /** Unique key used to identify the survey question (like an ID) */
  key: string;
  /** Question text */
  question: string;
  options : Array<SurveyOption>;
  showIf? : SurveyQuestionCondition;
}

interface SurveyOption {
  /** Value to be saved / used as key for this answer
   * Try not to change these!! They are referenced from ShowIf.question properties
  */
  value : string;
  /** Label user will see */
  label : string;
}

interface SurveyQuestionCondition {
  /** Corresponds to a LOWER INDEX (earlier) SurveyQuestion.key */
  question : string;
  /** list of {SurveyOption.value}s for SurveyQuestion specified in this.question
   *  if the answer to the question specified by this.question matches one of the
   *  strings in this.in, the quest this is attached to will be shown
  */
  in : string[];
}