Replacing template sections does not work

[Tsjippy]

I have a custom html block in my template with some sample content:

<td class="defaultText" mc:edit="body">
<h2 class="title">Primary Heading</h2>
<p>Sample copy. Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
Morbi commodo, ipsum sed pharetra gravida, orci magna rhoncus neque, id
pulvinar odio lorem non turpis.</p>
<h3 class="subTitle">Subheading</h3>
<p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit.</p>
</td>

Then using the PHP library I try to replace this:

$response = $client->campaigns->setContent(
            $campain_id, 
            [
                "template"  => [
                    "sections"  => [
                        "body"          => "<p>his is a test</p>",
                    ],
                    "id"        => $template_id
                ],
            ]
        );

But this does not work.

Am I right this only works for custom build premium templates?