Yes, it's safe to defrag. Only your HDD will be defragged.
I hope this won't come across as being rude, but would you please tell me where you got this information from?
My PC uses an Intel SRT setup as well. Because I am not sure whether SRT is smart enough to tell the difference between regular read/writes from those that are done during defragging, I simply go to the SRT console and turn SSD caching off. It literally is a 1 button deal from the Intel console.
And then when the defragging is done, I'll just click the same button and turn it on again.