This hadîth was related by al-Nasâ’î, as you have mentioned, as well as in another context. `Abd Allah b. Mas`ûd said: “I never saw the Prophet (peace be upon him) combine between two prayers except in Jam`(Muzdalifah), and on that day he prayed Fajr before its due time”. Please refer to Sahîh al-Bukhârî (1598) and Sahîh Muslim (1289).
Some people of knowledge, such as al-Hasan al-Basrî, Ibrâhîm al-Nakh`î, Abu Hanîfah and his two students, used these hadîth as evidence that combining prayers is only permitted in `Arafah and Muzdalifah.
They considered the other hadîth that permit the combing of prayers as a concession for as merely an apparent combination, just like what you mentioned in your question (i.e., delaying the first prayer until the end of its due time and advancing the second prayer to the beginning of its due time).
This opinion is weak, since it goes against authentic hadîth that clearly permit the combining of prayers for travelers. These hadîth discuss a clear combination of two prayers in the time of one and not the mere appearance of combining them.
Some of these authentic texts are:
1. It was related by Anas that the Prophet (peace be upon him) used to postpone Zuhr prayer until the time of `Asr when he traveled before midday. Then he would come down and combine the two prayers together. But in case he departed before midday, he would pray Zuhr before riding. [Sahîh al-Bukhârî (112) and Sahîh Muslim (704)]
2. It was related that when Ibn `Umar used to travel he would combine between Maghrib and `Ishâ’ after sunset and says: “When the Prophet (peace be upon him) used to travel, he would combine between Maghrib and `Ishâ’.
The majority of the people of knowledge did not give preference to the hadîth that indicate the Prophet (peace be upon him) never combined his prayers. They explained that `Abd Allah b. Mas`ûd merely related what he saw and recalled. Therefore, the reports of those who confirm that the combining of prayers actually took place – such as those of Anas and Ibn `Umar – must take precedence over the reports that claim it did not happen.
And Allah knows best.