Is the husband's saying to his wife: "I swear by God, if you don't return before 9:30, you will be forbidden to me until the Day of Judgment" considered a divorce if the wife returns after the specified time, knowing that the husband said that due to her stubbornness and disobedience?

If you intended that your wife would be divorced or that ẓihār would occur from her merely by her being late from the appointed time, then what you intended for the suspended condition to occur has indeed occurred, which is her being late. However, if you intended that you would divorce her or perform ẓihār from her if she were late, then this is a threat and a promise to do so, and you are not bound by it unless you actualize it. Your statement, "You will be forbidden to me until the Day of Judgment," is a phrase that could signify an immediate effect or a promise, and it could imply ẓihār, divorce, or an oath. This depends on your intention and purpose, and the matter should be presented to a judge to rule upon it.