The hadith doesn't explicitly say that 1500 sahabi made the ablution, but it says that this amount was present when this happened. So we can't exclude that some of them have been in a pure state. Note that scholars such as ibn Hajar discussed the amount as the reports seem to have a certain difference.
The hadith can actually be explained by another version of the same narration:
Jabir said "On the day of Al-Hudaibiya, the people felt thirsty and Allah's Messenger (ﷺ) had a utensil containing water. He performer ablution from it and then the people came towards him. Allah's Apostle said, 'What is wrong with you?' The people said, 'O Allah's Messenger (ﷺ)! We haven't got any water to perform ablution with or to drink, except what you have in your utensil.' So the Prophet (ﷺ) put his hand in the utensil and the water started spouting out between his fingers like springs. So we drank and performed ablution." I said to Jabir, "What was your number on that day?" He replied, "Even if we had been one hundred thousand, that water would have been sufficient for us. Anyhow, we were 1500.' (Sahih al-Bukhari)
Ibn Hajar commented the hadith (see here in Arabic) above in the chapter of the Ghazwa of Al-Hudaibiya that this is different to the hadith that al-Bukhari quoted earlier on the authority of al-Bara' saying:
That they were in the company of Allah's Messenger (ﷺ) on the day of Al-Hudaibiya and their number was 1400 or more. They camped at a well and drew its water till it was dried. When they informed Allah's Apostle of that, he came and sat over its edge and said, "Bring me a bucket of its water." When it was brought, he spat and invoked (Allah) and said, "Leave it for a while." Then they quenched their thirst and watered their riding animals (from that well) till they departed. (Sahih al-Bukhari)
As al-Bara' ibn 'Azib says that the prophet () poured the water from a bucket while that of Jabir says otherwise. He added that ibn Hebban could bring both ahadith in line by saying this happend at two different occasions, the statement of al-Bara' was more general, while that of Jabir was for the time of al-'Asr prayer. And he claimed that it is possible that the narration of al-Bara' happened after they have performed wudu'.
This was not the lonely time the sahabah witnessed such a miracle:
Anas said, "The Prophet (ﷺ) asked for water and a tumbler with a broad base and no so deep, containing a small quantity of water, was brought to him whereby he put his fingers in it." Anas further said, ' noticed the water springing out from amongst his fingers." Anas added, ' estimated that the people who performed ablution with it numbered between seventy to eighty." (Sahih al-Bukhari)
We used to consider miracles as Allah's Blessings, but you people consider them to be a warning. Once we were with Allah's Messenger (ﷺ) on a journey, and we ran short of water. He said, "Bring the water remaining with you." The people brought a utensil containing a little water. He placed his hand in it and said, "Come to the blessed water, and the Blessing is from Allah." I saw the water flowing from among the fingers of Allah's Messenger (ﷺ) , and no doubt, we heard the meal glorifying Allah, when it was being eaten (by him). (Sahih al-Bukhari)
See also in Sahih al-Bukhari on the authority of 'Imran ibn Hussain.
The explanation therefore is as the water spout, flow or sprung from the prophet's() fingers it is not necessary that they have used a single bucket, but they may have also used the well too. And as we are talking about miracles we may need either to accept the narration as is or consider that there might be an answer we don't know of. If Allah made blessings in the water the prophet () touched with his fingers He is also able to bless them with time to perform ablution and drink etc. from the same water.
Note that in some narration a description of the vessel/bucket/container was quoted as:
...بماء في صحفة ..
Scholars say that such a vessel can be used by five people at once.
In some narration the statement of Jabir is:
Al-Qustalani in his Irshaad as-Sari إرشاد الساري said this might be an indication for the amount of groups.