Long story short: Yes, it was to deter and halt a potential and much-feared German attack but no, that was most likely not the reason the Germans stopped.

It is true that upon the invasion of the Balkans by the Germans, Turkey destroyed the bridges between itself and its Balkan neighbours. With the exception of a small, picturesque town in Edirne called Karaağaç, Turkey is separated from Bulgaria and Greece by rivers. Therefore, by severing these links, Turks hoped to make their defensive intentions clear. More importantly, this is a pretty much textbook military tactic, especially against an army like the Germans which relied so heavily on armoured divisions, which in turn performed best when the ground was clear. Leaving the bridges intact would literally turn these into highways for German tanks to come and invade Thrace.

I am not a historian of the Wehrmacht or anything, but based on the general knowledge I have on WW2, I can say that the destruction of bridges alone would not stop the Germans. Surely, this would have made crossing the border more troublesome, but Germans certainly had the forces to overwhelm Turkey. Thus, it seems unlikely that the bridges being blown up suddenly led Hitler to decide to attack the USSR instead.

To my knowledge, why the Germans did not attack Turkey at all remains a bit mysterious. There are many potential elements at play here. One, Turkey has a quite mountainous terrain and despite the success in crossing the Ardennes in the Western front, you want your tanks and blitzkrieg in fields and on highways and roads rather than on top of a mountain or in a forest. The bridges blown up may have been effective here in signalling to the German high command that Turks did not intend to surrender quickly but would fall back while fighting (as much as they could do, at least). Second, remember that Germany and the Ottoman Empire were allies during the WWI. Many German officers served in the Ottoman Army then and most seems to have left the country positively surprised by the fighting spirit of the Turkish people. Initially, before the WWI, their opinion was largely negative: the ambassador von Wangenheim almost begged Kaiser Wilhelm II not to take the Ottomans as an ally! But the Ottomans did not fall as quickly or easily as the likes of von Wangenheim feared. Add to this the fact that unlike Germany, which signed the Treaty of Versailles, Turks fought against their Versailles (the Treaty of Sevres) and won their independence back. Stefan Ihrig has a whole book (Atatürk in the Nazi Imagination) about how this and the image of Atatürk exerted influence on many Germans at the time, including Nazis. The Turkey and Atatürk in the Nazi imagination, of course, was something quite different from what these really were; but that is not our subject. What matters is it is likely that due to such influences still current in the German armies, they expected Turks to resist and engage in guerilla warfare. Finally, and crucially, for a good part of the WW2, Turkey's relations with Nazi Germany were actually quite good. Turkey supplied important materials for the German industry, most importantly chromium. While initially closer to the Allies, Turkey under İnönü avoided entering the war until the very last moment. Keeping especially Britain close and avoiding conflict with the USSR, they also made room to have good relations with the Nazis. The result was a rather bizarre situation where both Nazis and the Allies tried in vain to bring Turkey to their side definitely, with Turkey seemingly enjoying good terms with all and anxiously waiting for the result of the war to become more evident. Once this was the case, Turkey gradually severed its ties with Germany and eventually declared war on the Axis, only in 1945. However, by the time Germans were taking the Balkans, they had no particular reason to think of Turkey as an active Allied country.

As a separate note, two of my grandparents were Turks born in Bulgaria and they were young children during the WW2. It sounds odd today, but they would both say the German armies and officers were much kinder towards Turks than the Soviet armies that came later. They were from different villages of different districts, so this suggests a general policy rather than a very localised oddity. I suppose this too was a part of the general German effort not to offend Turkey and try to win its friendship.

Germany could have perhaps invaded Turkey to reach the oil-rich regions of Soviet Azerbaijan and the British-held Iraq and Syria, but they did not. To the great relief of Turkish leadership at the time, they chose to attack the Soviet Union instead. As before, to my knowledge, there is nothing definitive on why Germany did not attack Turkey: I suspect it was the combination of the factors described above, but a historian of the Nazi armies could correct me if I am wrong. We can safely conclude, however, that it was not the blown-up bridges that saved Turkey alone. If the account thus far is right, we can also say that Germans did not cross the Turkish border. The only place they would do so without crossing the river was Karaağaç and, again, to my knowledge, the place was never invaded or sieged by the Germans.