While you can modify the AMP Reader mode templates to add the same code, probably a better approach is to inject the breadcrumbs with a the_content filter. Without the is_amp_endpoint() check even, this will also add the breadcrumbs to any theme as well so you don’t have to modify the theme files:

add_filter( 'the_content', static function ( $content ) {
	if ( 
		function_exists( 'yoast_breadcrumb' ) 
		&&
		function_exists( 'is_amp_endpoint' )
		&& 
		is_amp_endpoint() 
	) {
		$content = yoast_breadcrumb( '<p id="breadcrumbs">','</p>', false ) . "\n\n$content";
	}
	return $content;
} );

Putting this plugin code in the single.php file would probably not be correct. It would be better to put it in a separate plugin that you activate separately.

For example, create a file like wp-content/plugins/learn2love-amp-breadcrumbs.php and put in the file:

<?php
/**
 * Plugin Name: learn2love AMP Breadcrumbs
 */

add_filter( 'the_content', static function ( $content ) {
	if ( 
		function_exists( 'yoast_breadcrumb' ) 
		&&
		function_exists( 'is_amp_endpoint' )
		&& 
		is_amp_endpoint() 
	) {
		$content = yoast_breadcrumb( '<p id="breadcrumbs">','</p>', false ) . "\n\n$content";
	}
	return $content;
} );

Then activate the plugin.