Is it permissible to combine the Maghrib and Isha prayers due to cold weather and wind, for a traveler whose home is 63 kilometers away from the mosque and who returned to his home on the same day?

What the Imam did by combining the prayers is a permissible concession if the wind was strong and cold, causing hardship. However, if the wind was normal, combining the prayers is not prescribed. Otherwise, the 'Isha prayer is invalid and must be re-offered because it was prayed outside its time, unless separating the two prayers caused you hardship, in which case combining them is permissible due to need, as chosen by Ibn Taymiyyah.

The more cautious approach is to re-offer it due to the absence of the intention to combine when praying Maghrib.