Our account people are in the process of confirming that it will be supported at some point. I belive what you say when you say you got your confirmation but just wanted to share what jtac posted to me:
Hi Cord, After doing some research, I can confirm that this feature (ip-source-guard) is not supported on EX3400 platform. You can also check here: https://pathfinder.juniper.net/feature-explorer/search-features.html# EX3400 is not mentioned under the platforms that support this feature. For the time being there is no explicit plan on the roadmap of implementing this feature. There is an ongoing effort to include this feature in the future releases for EX3400, but there is no ETA for that yet. Please tell me how do you want to proceed with this case. Thanks Aaditya