He is referring to the terrain penalties you can see listed in the wiki (although it doesn't seem that the wiki gives the penalties AT has attacking through plains). So yes, attacking a hard unit with AT is still better than attacking a soft unit. But it also means that the penalty hurts the entire division, so attaching AT to a division drags down its overall values when attacking and not just the AT brigade's attack values. INF has generally better values at attacking into all kinds of terrain, and you could substitute the AT for something else, so the decision to use AT in significant numbers, and HOW to use AT in significant numbers, is a question you have to think long and hard about. Just spamming AT brigades and attaching them to all INF divisions may not be the best use of IC or time.
Thanks for the quick reply. Some follow-up questions:
1. Are the penalties listed in the Wiki valid for SF 2.04f or some other version?
2. What are the penalties for attacking in plains for the various units? or
3. Where could I go to find this info? Forum post? Wiki page? Game file?