Well, PHP 5.3.3 is needed for LimeSurvey, and XMLWriter is part of it by default
http://php.net/manual/en/xmlwriter.installation.php

For Gentoo Linux, as the link you provided so notes, XMLWriter is disabled by default:

Comment by alvaro at digdib dot com 7 years ago:

Gentoo users: emerge does not compile dev-lang/php with --enable-xmlwriter by default. To enable this option, add the flag xmlwriter to your USE variable and recompile php.

It appears Gentoo's package manager decided to keep XMLWriter disabled as the default option. Now the question is whether Lime Survey's documentation would cover such a variance. I had reviewed your installation documentation when I installed Lime Survey on my Gentoo system and did not learn of this deficiency until I attempted to submit a Bug and provide an exported survey, and even at that my export occurred without any error message and I neglected to inspect the contents before uploading the file to my Bug.

XMLWriter is a required library - you will need it even if you only want to copy a survey.

The documentation already says below the required modules:
"Also we assume in general that all PHP default libraries are enabled (like hash, session, etc.)."

Unfortunately we cannot list all exceptions for all distros - we assume that if a library is enabled by default in PHP it does not need specificially to be mentioned in the documentation.