Spec does not mention that lowercase should be ok, rather, to me it reads as if it should not be allowed: https://spec.commonmark.org/0.29/#cdata-section.
The “Block” version of commonmarkjs behaves correctly and does not allow lowercase, but because of the single regex to parse “inline” html, it’s allowed.

Input:

<![cdata[]]>

Yields:

<p><![cdata[]]></p>

Expected:

<p>&lt;![cdata[]]&gt;</p>