MSDN is not "wrong", but somewhat incomplete.

Most of WIC is often just an intermediary between your code and the decoder/encoders (codecs, specific for a given file format).

The fact GetMetadataQueryReader is supported on XP doesn't mean a given codec on XP supports the metadata with the same level as the same codec on other Windows versions. Images metadata can be quite complex and deep (and specifications can be improved in time).

If you go to this MSDN page: What's new for Windows 7 you'll read this:

Extended Metadata Support for JPEG, PNG, and GIF:

In Windows 7, WIC has extended its metadata support for JPEG, PNG, and GIF images.

• Added support for animated GIF and GIF properties.

So it probably means you're touching GIF files that have unsupported metadata properties, for which support was added post-XP (Vista or 7).