Why only use the solar sail? Use the sail to get up to speed, then disengage it and switch over to using a Bussard ramjet. Use of the ramjet should provide ample power for guidance corrections using a vectored thrust system.
Of course, the article may be leaving out a point like using gravity boosts for the sail on its way out, in which case a thousand kilometers would be a very drastic course deviance.
Multi-tasking is not the same thing as multi-threading.
Two is not equal to three, even for large values of two.