Actually, you are talking to someone who dislikes using contractions.
So, I would be far more likely to say, could have, rather than could've. (Could've being the contraction of could have) In fact, I may not have used that one in decades.
The same goes for couldn't and could not. <<<<Used on rare occasions.
To me it always seemed to be lazy to use many of the contractions.
Can't being one of the few that I do sometimes use, but even then, I will also use can not.
Everyone speaks differently, and something like what Lin did is so incredibly minor, that it could easily be something regional.
The use of "could of" is mostly in the United States, making it more of a regional use for it.
Grammatically correct? Nope, but it is common enough in the U.S. that it is accepted by everyone except for perhaps people who have attended higher learning centers, such as colleges.
You can't expect street hoods to speak the language well.
So if you have a story with them in it, are they supposed to all speak like James Earl Jones?
Because it is the proper way for them to speak.
Or, does the game developer type out the text of what they would say, as they would say it?
Even though what they are saying is absolutely horrible grammar, but it is, how they talk.
Because for a writer to tell a story, even bad grammar has its place, if it fits the story and gives it the proper feel when it is read.