A guided munition has control surfaces thait can cause course correction. Not all guided munitions can hit moving target or used for air-to-air combat. Most smart bombs fall into this category.
Looking at this weapon, it is clear that the fins can rotate about an axis, allowing for in-flight course correction. Also, let's look at what the article says in full on this:
Guidance. The HVP is claimed to be guided but that’s true only in a limited sense. The guidance is GPS and is applicable only against fixed, land targets with known GPS coordinates. Useful guidance is not possible against moving land targets or aerial targets due to the extreme speed of the projectile.One of the “side effects” of speed is inertia. The faster an object moves, the slower and harder it is to alter its course. Faster means a larger turn radius. A WWI Fokker Triplane has immensely greater maneuverability than a modern F-16 because the F-16 has such high speed. An HVP traveling at Mach 7 cannot easily change course. An incoming cruise missile traveling at high subsonic speeds, for example, would be far more maneuverable than a Mach 7 HVP which is, for practical purposes in that scenario, ballistic and non-maneuverable.
What the author is pointing out is not that it is not guided, but it is not as maneuverable as the subsonic cruise missile. He says it is not easy to change course, not that it cannot change course. This would still allow for level flight, depending on how the control system works.