The obvious answer is the right one; he just likes a big handful of gummy bears after a race. Probably does the same in training too. The gummy bear as optimal recovery food has been around for years, it's not new. Bodybuilders have advocated it for years. When I was racing it was Jelly Babies. Sagan is just a gummy bear man. A hairy gummy bear man.
Whether he had hunger knock or not at end of the race is a red herring. Soigneurs usually have a little backpack with the various things their riders want/need at the end of a race. No advance communication needed. Many want a can of coke. Some want recovery shake. Sagan likes gummy bears.
The drug masking idea is tinfoil-hat stuff. Very unlikely top riders nowadays have enough doping product still in their system to be at risk of failing post-race testing. Too high a risk. Controlled micro-dosing in advance, maybe plus masking agents, is what they are doing.
Even if he took such a stupidly high risk, the gummy bear idea as masking agent seems incredibly silly. Fastest absorption is surely liquid, sublingual or ingestion, not swallowing gummies. Never mind that these gummies laced with masking agents would have to be specifically made by someone in the team, made to look like Haribos, tested to see how effective they are etc. Pointless waste of considerable time. And gelatin alone might work at masking as in that Cannabis link, who knows, but certainly not in the very short time window before the post-race test. Our metabolism doesn't work that quickly.
What is more interesting to me is that this is even being discussed. Indicative of just how little trust there is in riders now, suspicion over absolutely everything. Probably justifiably so, unfortunately. This gummy bear thing is by far the maddest suspicion I've seen so far though.