I guess if the RPM is kept low they grease will not evaporate too quickly.I think you have to grease the valves every so-and-so miles, or it could be like a diesel locomotive where oil is very gradually allowed to seep out onto the components.
These are just guesses, though, so I could be wrong (someone who knows more about Brutus correct me).
As in more modern overhead cam engines, the tappet faces/top of valve stem face are not directly lubricated, and wear is accommodated by tappet adjustment and/or bucket and shim on more modern engines. The valve stems on pre-80?s petrol engines normally ran in phosphor bronze guides and were lubricated by the lead in petrol, however over time the guides (and valve seats) hardened so high mileage ?leaded engines? can run fine on unleaded petrol with no noticeable increased wear in the guides or valve stem. The camshaft is fed an oil supply as shown in the photo via supply and return feeds, and this also lubricated the rockers. Shown in the photos the actual camshaft is enclosed whilst the valve tops are exposed. In more modern engines the camshaft still has an oil supply but the cam cover or rocker box encloses both shaft and valves.I guess if the RPM is kept low they grease will not evaporate too quickly.