"they filled the empty holes with solder to make it harder"
Whilst that may have been the outcome, I doubt it was the intention. For through-hole PCBs which are being wave soldered, the only reliable way to prevent empty holes from being bridged by solder is to temporarily cover them with kapton tape or similar. As this is an additional manufacturing operation which costs both time and materials, and thus ultimately money, it's the sort of thing which tends to be done *only* if those empty holes need to still be empty after the wave soldering has been completed - e.g. to allow components to be fitted to the underside of the PCB.