Best place IMNSHO to check is @ www.avsforum.com. The expertise there is great for this really nuanced type of question regarding a specific model feature comparison.
My gut feeling says you are correct, but it is actually the Sam's Club version of the 6400, which seems to only lack the 'ac' wifi specification of the 6500 models (and higher).
Also, if you are locating this HDTV in a dark basement- your situation is ideal. You very well may be able to get by with a slightly cheaper HDTV that can perform extremely well (contrast-wise) in a 'man cave' environment. Huge reflective screens in that size range drive me nuts in upstairs/typically bright spaces- even with good blinds. Absolutely ruins the good picture quality they are capable of(!) 4K resolution and HDR cannot fight natural lighting
I am in the process of finishing my basement and putting in a surround sound system. My last item is to select a TV and I am focused on 75" and think I have narrowed in on Samsung.
The person helping me with AV recommended the Samsung 75" UN75JU5600 which is selling on Amazon for $3,358 and has received great ratings. Link: http://www.amazon.com/Samsung-UN75JU6500-75-Inch-Ultra-Smart/dp/B00TWFI0RO/ref=cm_cr_arp_d_product_top?ie=UTF8
I am trying to compare that TV to the one I saw at Sams Club for $2,498 that is also a Samsung 75" (UN75JU641DFX2A). Link: http://www.samsclub.com/sams/75-uhd-smart-120hz-3-hdmi-wifi/prod19540478.ip?navAction=push. Price is not on website but I was just there and that is what it is selling for.
My AV guy says that is a TV made only for big box stores and appears to be the 6400 series but made in a 75" size only for Sams. He also said it likely has cheaper componentry inside of it? I am trying to reconcile the difference between these two TV's as the price diff is significant. Is it worth spending the extra $800+ for the 6500 model? Any guidance is greatly appreciated.