For simple modules your statement is true. When converting projects to Mendix 5 some migration steps are required. For example the 'java migration' wizard step. Those steps aren't executed on content you download from the appstore.
So, when downloading a complex module from the appstore, there is a big chance the module will not work in Mx5. As there is no 'complexity' differentiator attached to a module, all modules are treated the same and no download button is available in this situation. If a download butoon would be nade available, a user will be annoyed because he/she's able to download the module and it does not work.
Please contact the creator of the module to have it available for your desired version.
I am working on an update for this module with nationality according to the Dutch NEN norm. Time is the issue at the moment to finish this. But otherwise I will publish a 5 version as soon as possible to circumvent this issue.
Regards,
Ronald
[EDIT] As a workaround you could export the module in 4 and import it in the 5 version.