On their about page: >OpenStreetMap has a list of (nearly) every bench in the world – but they don’t record whether there is an inscription on it. Weird since OpenStreetMap does have an inscription=* key, which could be combined with the historic=memorial and memorial=bench tags. So what the site does could very well be achieved with OpenStreetMap alone.
I understood that to mean not that OSM can't record that information, just that it doesn't - I assume people tend not to include that level of detail when contributing to OSM. So the benefit of this site is that it focuses contributor's attention on the inscription and prompts them to include it. It's a cool idea - I wonder if they feed the data back up to OSM. If not they (or someone) presumably could easily enough.…
So in OSM, it's not that it can't record, or that it doesn't record. It's always that not many mappers haven't yet focused on the topic.