Every single muscle group in the body is a mixture of fiber types. The fiber type that predominates depends on the function of the muscle. The ratios can vary somewhat among individuals but not to a large degree. If they did then your body simply wouldn't work properly.

Slow twitch fibers need a high rep range to be stimulated fully. They are the endurance fibers. Seated calf raises primarily work the soleus which is predominately slow twitch. Reps in the 3 to 5 range would bypass the slow twitch muscles completely and only work the 10% or so of fast twitch fibers. Leaving out a whole lot of potential growth. Going slow will help but there is a threshold and once you reach that in weight the fast fibers come into play regardless of the speed of the movement.

By the same token, 1 set of 35 reps would only recruit the slow twitch fibers not induce any growth, IMO.

And that is not even getting in to the fact that seated calf raises don't work the gastrocs very well at all. The leg needs to be straight to fully employ and contract them.

I know there is an idea that you can get fast twitch fibers to behave a little more like slow twitch fibers. I.E. increase their endurance. But you are not gonna get slow twitch fibers to act like fast ones.

I'm simply amazed that that prescription worked for you, larryjoe. If I trained my calves with those parameters and that frequency I'd have sticks.