There are flat tappet cams in both the 66 Mustang I gave my grandson and my 76 Vette and they both run Penz hi mileage and no additives. I rebuilt the 289 in the Mustang in 79 and put a healthy Comp cam in about 10 years ago. I will replace the cam in the Vette later this year and I'll zddp for break in and after that I'm not sure.
I personally think it's more to do with the quality of the parts and too heavy spring pressure than the lack of zddp.