Full quote:
(On the possibility of countering the Allied invasion by using tanks)
As it was, the whole burden rested on the Army (Heer), and it was no longer "first class." The infantry divisions were weak and practically immobile, particularly their artillery and supply services. What panzer and panzer grenadier divisions there were, were used only piecemeal. This was a terrible error of the High Command; the fault lay partly with Hitler, partly with OKW, and of course, with von Kluge as well.
edit: His original words are "a terrible error" as indicated in the full quote, not a "horrible mistake" as in the title, I mistranslated that from my own translation, my bad.
No, Guderian is wrong.
There are multiple factors you have to bear in mind during the Allied invasion of France. First, the allies had major advantages in materiel. By 7 July 1944 the British and Canadians had a 4:1 superiority over German tanks in thier sector. In June 1944 the Luftflotte 3 (Luftwaffe for the English channel) had 319 servicable aircraft. The Allies had 12,837. This included numeorus "fighter-bombers" like the Hawker Typhoon which could carry, albeit ineffective, rockets. When operations were close to the Channel the allies also had naval gunnery they could rely on. Together this meant that the Germans could only move tanks by night. If large formations moved during the day time, they would be decimated. (Main reason the Battle of the Bulge went so well initially was heavy fog prevented Allied aircraft from intervening).
Second, the German logistical system was collapsing. In 1940 the Germans were already short of supply trucks. When they invaded the Soviet Union they possessed 120,000 - 5,000 understrength and suffered average losses of over 2,400 trucks per month with only 1,000 truck being produced each month. This meant the Germans were reliant on horse drawn vehicles right up until 1945 and meant that during the Allied invasion of France their supply routes were constantly overstretched defending the territory they had, never mind going on the offensive. Again, going to the Battle of the Bulge, one of the main reasons it broke down was an inability to keep the frontline units supplied and them subsequently raiding Allied stores.
Third, the supposed "armour gap" or apparent superiority of German tanks was simply untrue. Whilst the Tiger 1 and Panther were arguably better than their Allied rivals like the Sherman, they were present in far lower numbers with the Allies having roughly a 3:1 superiority. Only 5% of the 2,500 German tanks in Normandy were Tigers. And were by no means invulnerable. Panthers could easily be taken out from the side, and artillery, anti-tank guns, and planes could all destroy Tigers. The majority of German vehicles were Panzer IV's and Stug III's which, whilst effective, were firmly on an even keel with the more numerous allied tanks. With both of them being able to take out the other.
Fourth, Normandy is some of the worst ground possible to fight tank battles on. This is because it consists of "Bocage" which is basicall many small farming fields seperated by raised mounds and hedges, all of which are incredible difficult for tanks and infantry to cross. This meant the scope for manouver was severely constrained, unlike the open plains of Eastern Europe.
Together this meant that is was far more effective for the Germans to use their tanks on the defensive as it meant they could be positioned at night, protecting them somewhat from Allied fighter-bombers etc. It meant they could ambush Allied forces (beneficial for obvious reasons). It meant they could keep their better tanks (Panthers and Tigers) at a range at which Allied weapons struggled to penetrate them. This also ensured German tanks had the best chance of "shooting first". And it forced the Allies to advance over bocage which was inherently tricky.
Lastly, the idea they were deployed piecemeal is wrong. They still managed to mass tanks, such as at Operation GOODWOOD on 18 July where 400 German (a significant portion of those available) tanks faced 1,350 British tanks.
Conclusion, Guderian is wrong. Also remember Guderian was a tank theorist, and you don't meet many tank theorists who said anything other than "THE TANK IS THE ONLY ANSWER" at this point a la Basil Liddel Hart (who actually blackmailed Guderian with the threat of execution into saying BLH influenced blitzkrieg) and JFC Fuller.