Hi Syd,
Those ellipses are really doing a lot there 
They literally snip out ”in the context under discussion”, ie it’s being sincere with regards to the topic of that discussion, not as a general thing.
He even says it later ”Anybody can be sincere (about anything) – all it takes is seeing the fact (of anything) […]”
Being sincere basically just means, seeing the fact (actually seeing the actual fact, as in, what is really/truly/factually/actually the case (using all these as the standard dictionary definition synonyms)) and being in accordance with that fact, ie not trying to deny it (to oneself or others) etc.
Ultimately it comes down to not fooling oneself… about anything! … and when actuated, such sincerity really does wonders!
Cheers,
Claudiu