Interestingly this can be answered from the first sugya in the gemara there.
אמרי דבי רבי ינאי לרבי יוסי הגלילי שמעינן ליה דאמר אפשר לצמצם בידי שמים וכ''ש בידי אדם ורבנן בידי שמים אי אפשר לצמצם בידי אדם מאי
Now, although this initially seems to sound more impossible, being that we attribute more ability for tzimtzum to people than heaven, the fact of the matter is, this is not true.
What the sugya calls 'in the hands of heaven' are acts within nature such as the example there, an animal giving birth. What the sugya calls 'in the hands of man' refers to an act done with conscious intention.
Therefore Hashem's tzimitzum was certainly able to be performed. Firstly it was a conscious act. Secondly nature was not created yet so there was no 'in the hands of heaven' as referred to in the gemara.