Here is an explanatory paragraph from what is apparently Steve's original site:
To help further explain this, I'll give some examples using hypothetical lactate values. If athlete A before he starts training for the mile has an anaerobic capacity of 16.0mmol (his max lactate value) and a LT pace of 5 minutes per mile, then he trains using high amounts of LT work the following could happen. The LT work could lower his pace to 4:55 per mile which would be good, but it might also lower his anaerobic capacity, or max lactate value to 12.0mmol. So if he goes out and runs a mile he might actually race worse than before even if his aerobic system is better, because his anaerobic system has been neglected and won't "go as high."
Link to specific page
So the claim that he is putting forth is that by training at our near your threshold for an extended period of time you "lower the bar", so to speak, of lactate tolerance. So if you lower the lactate tolerance, or the point where aerobic and anaerobic start meeting, it would negatively impact performance.
Note, this won't really affect anything under 400m, and will have not much effect at the 400. It also won't really affect anything greater than the mile, except possibly in sprint finishes. The major impact for this is going go be in the 4-800 meter up through the mile (ish). (If you believe the theory, I haven't researched it enough to see if there are corroborating studies for the above claim).