This is true if you have some antiquated definition of broadband that includes a competitor offering "up to" 896kbps while Comcast offers everything from 1 to 20+ mbps, but false for everybody else.
The cable industry has evolved into a bunch of regional monopolies that do not compete with one another.
The effect of breaking Comcast up AT&T-style would be... a bunch of regional monopolies that do not compete with one another. And then, as with AT&T, proceed to consolidate via M&A into larger regional monopolies... that do not compete with one another. Same as now.
Welcome to the American telecommunications industry.