If it is new white pine you are right that it will be soft, but in that time period, it was common to use heart pine for flooring. It is extremely hard and strong. It was often used directly over the joists. It took wear well enough to be chosen for installation in public buildings such as schools and courthouses. It is similar in appearance to clear vertical grain doug fir flooring you can buy now. rubbing, sanding, or cutting it can release a turpentine smell.
If it is heart pine in decent shape, it is a rare and valuable find. It will gum up sanding pads easily though from the pitch in the wood.
It comes from the long leafed Lobblolly pine and is not commercially available anymore. There are only about 100,000 acres in plantation growth in the country. Old growth virgin trees are long gone and so is their heart wood, except for salvage operations.