Integrating slick.js into wordpress

Adjusted some of your code. This should work:

<?php

/**
 * Block Name: Testimonials
 *
 * This is the template that displays the testimonials loop block.
 */

$argType = get_field('loop_argument_type');
if ($argType == "count") :
    $args = array(
        'orderby' => 'title',
        'post_type' => 'testimonials',
        'posts_per_page' => get_field('testimonial_count')
    );
else :
    $testimonials = get_field('select_testimonials');
    $args = array(
        'orderby' => 'title',
        'post_type' => 'testimonials',
        'post__in' => $testimonials
    );
endif;


$the_query = new WP_Query($args);

?>

<section class="testimonials">
    <div class="container">
        <div class="row">
            <div class="col-md-12">
                <div class="testimonialHolder">
                    <?php
                    if ($the_query->have_posts()) {
                        while ($the_query->have_posts()) {
                            $the_query->the_post(); ?>
                            <div class="item">
                                <div class="testimonial">
                                    <?php $image = get_field('foto', get_the_ID()); ?>
                                    <img src="<?php echo $image['url']; ?>" class="img-circle" alt="<?php echo $image['alt']; ?>" />
                                    <h4><?php the_field('naam', get_the_ID()); ?></h4>
                                    <p><?php the_field('samenvatting', get_the_ID()); ?></p>
                                </div>
                            </div>
                        <?php } 
                    } else { ?>
                        <div class="item">
                            <div class="testimonial">
                                <h3>No Testimonials Found</h3>
                            </div>
                        </div>
                    <?php }
                    wp_reset_postdata();
                    ?>
                </div>
            </div>
        </div>
    </div>