I think it's a case of making too many promises and being something they ultimately couldn't deliver rather than lying from the outset. I wouldn't be surprised if they simply canned heists. I figure they were too complex in structure to work properly online, and dumbing them down from how they work in singleplayer wouldn't be received very well. Or particularly fun either.
Happy to be proven wrong, though I'm not sure I care about them at this point.
I think originally the plan may have not been that big in scope... I mean, we never really ever saw what Online Heists were supposed to be. Rockstar almost never officially talked about them. It was mostly just IGN adverticles and the like. Only real official mention of them is that official GTAO gameplay video and it basically just shows a few seconds inside Pacific Standard bank following a waypoint, and it does that in the context of describing online Heists alongside robbing cashiers and also team deathmatch -- both of which suggest a significantly lower standard for MP heists than SP heists. And the other originally planned MP heists sort of did make it into the game. Parts of that MP heists where you 'heist' a titan did end up making it into GTAO (similar how the Madrazo heist became Caida Libra mission).
As expectations grew for GTAO as a whole and specifically MP Heists, Rockstar probably recognized that their couple very basic and simplified mission-like Heists were going to be underwhelming and boring rather quick. If I had to guess based on the delays, they probably rebooted their Heists system (more non-linear? random heist generator?) because the Pacific Standard heist that was originally hinted at in that 5 second clip (the only real official preview of GTAO Heists) probably could have been finished in barely a month of development, and yet 12 months later we're still waiting.
They've made so many other missions and even two pretty decent map/mode editors in than span... a single Heist mission shouldn't take too long so I would have to guess the scope of it is bigger or at least rebooted from that.