The index.php
file is the fallback template file when no other templates have been found. For a FSE theme this should never be run except when Gutenberg is not activated.
<?php
// Apparently this can be empty.
_e( 'Activate Gutenberg to use the "This is" theme', 'thisis');
?>
<?php
_e( 'You also need an index.html file in block-templates', 'thisis');