Grease on a carbon fiber seat post in a 4130 chrome moly frame?
 

I greased one a while back without thinking about it much. I just wanted to keep it from getting stuck.

Then the other day I read in Park Tool's Big Blue Book of Bicycle Repair that it is "not recommended" to grease a carbon fiber seat post. Is this because it is not needed or because the grease will harm the seat post?

Carbon seat posts need to be greased. There have a been a few threads where people have had to destroy the carbon seat post to get it out.
They may be referring to the tendency of carbon posts slipping due to the reduced seat post bolt torque recommended for carbon seatposts.
Use carbon assembly grease - it has grit in it.

I used a moly grease. Bad?

If it doesn't slip, don't worry about it.

+1, the only argument against grease on CF posts is that it reduces traction which is bad enough already. If the post stays put, you're in good shape. If you find slippage to be an issue you can revisit this, and explore some of the many solutions.

BTW- WARNING, be careful about going crazy with clamping load to secure the post. That's a good way to crack the clamp, or the ears which are part of the frame. It can also crush the post if you have a steel frame with ears built in. Overtightening digs the tube on either side of the slot in harder than the average clamping load, causing an effect like digging your fingers into an eggshell rather than squeezing it evenly. If you have a separate clamping band, apply grease between the band and frame, and rotate so the slots don't line up. That prevents the digging in action I described.

There's specific products to prevent this, and DO NOT use the typical grease in a garage. (white lithium, wheel bearing, etc.) Those greases can degrade carbon fiber. It has to get past the resin, but it's possible and you don't want to find out on a ride.

Use FSA fiber grip or anti-seize without copper. Solely synthetic grease can be used as well, like SuperLube, but the post might slide in every time you loosen it.

+1, it is very fine grit and you will find many uses for it, today I put a tiny bit on the mating surface of 1984 Cinelli quill and handlebar knowing it will never slip again even though I used less torque than normal.