HR is wrong

Once your H1B is approved, your F-1 is gone. There is no dual status here.

Even with OPT, you need to pay SS and med care tax. Some places may think it deferently, but it is not correct. (I assume your are in F-1 longer than 5 years).

If you have been in the US for more than 5 years, you should file tax return as a resident.