Apparently, the actual recommendation was to first replace the 4.163 Windows clients in use (77 % Windows 7, 9 % Windows XP/Vista, 14 % Windows 2000 [!!!]) with a single improved configuration, likewise for the different versions of their LiMux distribution.
Afterwards, each department should choose between the Windows and Linux clients based on a cost-benefit calculation. Only in case the Windows client ends up dominating, a reevaluation of the economic viability of the duplicated infrastructure is recommended.
Additionally, the only mentioned software problems from their user survey were caused by varying browser versions (presumably Firefox), their mail program (presumably Thunderbird) not being available on mobile and frequently crashing, the same complaints about their calendar software (no idea what they are using) in addition to lacking copy-paste functionality.
Unless LibreOffice has a calendar component, I don't think anyone complained about it? At least it isn't in the report.