I agree with Bob on the other site.
No one can draw a perfectly straight line without using a straight edge, like a ruler. The line "I can barely draw a straight line" is therefore illogical to start with.
The person who wrote this is admitting that drawing a straight line is actually very hard, not something that is the least someone with little talent can do.
I suppose "acceptably straight" might be correct, but the original sounds quite natural.