I've never seen StartupSettings.ini and I don't even have that file so it can't be anything useful.
Are you sure that's a MusicBee file - searching on the forum shows no results for that name.
As I stated when I mentioned the last modified datestamp on my StartupSettings.ini was 2014, a font and a theme are in the file. That coupled with a number of appearances of "MusicBee" kind of proves it's a file that MB used at one point.
<?xml version="1.0" encoding="utf-8"?>
<CoreSettings xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<SettingsPath>G:\MusicBee\AppData\</SettingsPath>
<LibraryPath>\MusicBee\phred\</LibraryPath>
<KnownLibraryPaths>
<string>\MusicBee\phred\</string>
</KnownLibraryPaths>
<PromptLibrarySelection>false</PromptLibrarySelection>
<DefaultFontFamily>Kabel</DefaultFontFamily>
<DefaultFontSize>9.749999</DefaultFontSize>
<DefaultFontStyle>Regular</DefaultFontStyle>
<Theme>Blue\Dem Ol' Kozmic Blues</Theme>
<UseNativeWindow>false</UseNativeWindow>
<PlayerAtTop>true</PlayerAtTop>
<ProgressBarLocation>3</ProgressBarLocation>
<PlayerLayoutFlat>true</PlayerLayoutFlat>
</CoreSettings>