syntax files not working in plugin directory

I've been using this very minimal syntax highlighting
scheme for Toki Pona,
its just an ftdetect which
sets the ft=tokipona
and a syntax file that is loaded by vim when
ft=tokipona:
https://github.com/ttm/tokipona

I like to keep the ftdetect/ ftplugin/ and syntax/
inside ~/.vim/plugin/tokipona/
(such as by doing
$ git clone https://github.com/ttm/tokipona ~/.vim/plugin/
)
For some reason,
the syntax file is only loaded if inside
~/.vim/syntax/ (not if inside ~/.vim/plugin/tokipona/syntax/).
But both the ftdetect and ftplugin are triggered inside
the plugin directory as expected.

Am I missing something?
The syntax file should be loadable from
the plugins directory, right?
How?

+tx,

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