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?