With any age, those screws are probably rusted, as are the receiving nuts. They are not holding down the threshold as much as they are there to adjust it up and down. They are captive, meaning they do not remove. Will they turn at all? Will the threshold wood move up and down with the turns?
You may be due for a new complete threshold.
MD Building Products - 4-9/16 in. x 36 in. Satin Aluminum and Oak Adjustable Threshold - Use this threshold with a door sweep to get a weather-tight seal, making your home more comfortable and energy efficient. - THD SKU# 254369
www.homedepot.com