You exported the wrong thing. The importer expects the Trello JSON you get from exporting a single board (via the board menu). If you export the entire workspace instead, the JSON schema for checklists differs and the importer crashes with TypeError: data.checklists is not iterable. The reporter confirmed that re-exporting the board individually made the import work, including checklists and comments. So do not use the workspace-level export; export each board separately and import those files.
Source: https://github.com/linear/linear/issues/622
Source: https://github.com/linear/linear/issues/622