Most languages have integrated spellchecking checking as a "grammar" rule, and well-developed languages have a much more developed spellchecking than the "vanilla" Hunspell dictionaries.
Since most people do not deactivate Hunspell in LibreOffice, it would not make sense to add that extra rule.
In order to take advantage of this feature you will have to activate it in LanguageTool plug-in options.
Note that, inside LibreOffice, all errors detected by LanguageTool rules (including spellchecking errors) will be underlined in blue.
There is already a bug report/improvement suggestion in LibreOffice page, but it didn't get enough support to have their developers look into the issue.