Replace post’s “the_content” with ACF value

Instead of updating the post’s content, you can filter it using the the_content filter.

add_filter( 'the_content', 'wpse241388_use_acf_field' );
function wpse241388_use_acf_field( $content ) {
    return get_field( 'article_text' );
}

Update

To apply to only your article post type:

add_filter( 'the_content', 'wpse241388_use_acf_field' );
function wpse241388_use_acf_field( $content ) {
    if ( is_singular( 'article' ) ) {
        $content = get_field( 'article_text' );
    }
    return $content;
}

References

• the_content filter
• is_singular()