The default doxycfg template generated via

doxygen -g myTemplate.doxycfg

states the following guidance for the two mentioned tags:

# The DISABLE_INDEX tag can be used to turn on/off the condensed index (tabs)
# at top of each HTML page. The value NO (the default) enables the index and
# the value YES disables it. Since the tabs have the same information as the
# navigation tree you can set this option to NO if you already set
# GENERATE_TREEVIEW to YES.

DISABLE_INDEX          = NO

# The GENERATE_TREEVIEW tag is used to specify whether a tree-like index
# structure should be generated to display hierarchical information.
# If the tag value is set to YES, a side panel will be generated
# containing a tree-like index structure (just like the one that
# is generated for HTML Help). For this to work a browser that supports
# JavaScript, DHTML, CSS and frames is required (i.e. any modern browser).
# Windows users are probably better off using the HTML help feature.
# Since the tree basically has the same information as the tab index you
# could consider to set DISABLE_INDEX to NO when enabling this option.

GENERATE_TREEVIEW      = NO

I would understand it in the following way: If GENERATE_TREEVIEW is set to YES, the information provided by the index is already visible, therefore one could DISABLE the INDEX, by setting this tag to YES.

The adjusted comments would be:
'Since the tabs in the index contain the same information as the navigation tree, you can set this option to YES if you also set GENERATE_TREEVIEW to YES.'

and

'Since the tree basically has the same information as the tab index, you could consider setting DISABLE_INDEX to YES when enabling this option.'

If this is a misconception or logical flaw on my side, feel free to just close this bug :)