It is Intel's memory controller, and ultimately the platform was originally a server grade system ported and rebranded for consumer use.  There are 16GB DDR4 modules in existence, Newegg's Business website has them for $240 per module, so Intel was able to test those out to create their RAM support values, they aren't making up numbers on the hope it works when the modules finally appear.  Business buy dozens to hundreds of DIMMs or their systems, it isn't like their wallets are crying when they make that money back within weeks.
 
But consumers don't usually make money with their systems, so it always comes down to costs.
 
You are right; in order for EVGA to support anything they have to have tested it out first so that they can help customers when they do the same and run across problems.  But going into EVGA's support section up top, it doesn't look like they've actually tested 16GB modules (totaling in 64GB in quad-channel) yet-- it could change when DDR4 prices drop since right now, eight 16GB modules will set you back $2,000.
 
Until then, it is better to go for combinations that most of their customers try out, which maximizes their support.   If you'd like a specific answer by an EVGA rep, you ought to call them up.