I'm fairly certain this has more to do with the average type of person that doesn't go to college rather than any causal link between Freshman Composition and good writing skills.
The truth is for 99% of people college is the best option for them. They'll get a job paying a decent salary, with a better work environment and the possibility of upward mobility if they so choose. Better than had they not gone to college.
There's a small, no miniscule, fraction of people who are motivated enough that they can achieve the same or more without the institutional learning and degree.
* I never finished college but benefited from my 3 years spent there.
I received AP credit and thus passed out of Freshman Composition so my experience is slightly different. In my AP class, everyone's writing skills improved but no one who was a bad writer became a good writer.
If you ask any author the most effective way to become a better writer they'll tell you to read. Most people don't like to read and thus (I believe there is a causal link here) most people are bad writers. It takes much more than two semesters of Freshman Comp. to radically improve ones writing skills. Quality writing is something that you internalize over many years.
A knack for sentence flow isn't practiced, it is absorbed. Want to be a good writer? You need a vocabulary to match. Those of us who acquired extensive vocabularies through osmosis can tell when a lesser person has whipped out the thesaurus in an attempt to sound "smart." (usually, the fancy words they try to insert just end up hilariously misused, since they don't understand the different shades of meaning attached to them. You can only get that through observation of the word in its natural setting.)
As anecdata, I know someone with an English degree from our flagship state school who can't manage to comprehend the difference between "its" and "it's," "your" and "you're," and routinely mangles grammar in a manner absolutely horrifying even before you take into account the fact they studied English for OVER FOUR YEARS and still never managed to grasp the proper use of the possessive.