The fact that there is any warning at all probably has to do with the fact that the mask's rating is only FFP2 and it can only filter 94% of dust particles, whereas other masks can filter 99% or more. FFP2 also only requires <8% inward leakage, which seems like quite a significant amount of leakage.
The reason the warning specifically mentions hardwoods may have to do with the fact that hardwoods are generally more toxic, as Daniel B. commented. A collection of points from a University of Illinois at Chicago paper state:
- Contact with the dust of many hardwoods can cause conjunctivitis (eye inflammation), hay fever, asthma, coughing, and other respiratory diseases.
- Some hardwoods can cause hypersensitivity pneumonia (alveolitis), and frequent attacks can cause permanent lung scarring (fibrosis).
- Some hardwoods contain chemicals that are toxic.
- Inhalation of hardwood dust is associated with a particular type of nasal and nasal sinus cancer (adenocarcinoma). This type of cancer has a latency period of 40-45 years, and occurs to the extent of about 7 in 10,000 among woodworkers who are heavily exposed. This rate is many times higher than the rate of nasal adenocarcinoma in the general population. Over half of all known cases of this type of cancer are found in woodworkers.
Yet the paper considers softwoods much less hazardous:
Softwoods do not cause as high a frequency of skin and respiratory
problems as do hardwoods. A few individuals can develop allergic
reactions to some softwoods.
Regardless of the higher rate of toxicity among hardwoods, any fine dust is hazardous and can cause respiratory ailments in sufficient quantities.
Keep in mind that most disposable masks cannot create as good a seal with your face as reusable respirators with replaceable P100 (or equivalent) filters, which filter 99.97% of particles 0.3 microns or larger.