Praise be to Allah.
We have previously discussed the hadith referred to in your question, in the answers to questions no. 170090 and 328440. With regard to your question about how this immense reward for this small action can make sense, the answer is:
Firstly: the bounty of Allah is immense, for He is the Appreciative (ash-Shakoor) Who gives immense reward for small actions. Hence in some narrations of this hadith, it says that the Prophet (blessings and peace of Allah be upon him) said: “And that is easy for Allah.” Narrated by ‘Abd ar-Razzaaq in his Musannaf (5570).
Ibn Hajar al-Haytami said in Tuhfat al-Muhtaaj (2/471): It was said that there is nothing in the Sunnah, in any saheeh report, greater than this reward, so pay attention to it. End quote.
Al-Qaari said in Mirqaat al-Mafaateeh (3/1035): One of the leading scholars said: We have not heard in the teachings of Islam of any saheeh hadith that refers to anything like this reward, meaning: one should make sure to comply with it in in the hope of attaining that reward. End quote.
So there is nothing strange about this reward, because no human can comprehend the grace and bounty of Allah. What is strange is our heedlessness concerning its, when we know that it is proven; look around you and see how heedless people are. And it is Allah Whose help we seek.
Secondly: this immense reward is only attained by doing all the things that are mentioned in the hadith: doing ghusl, cleansing oneself, going to the mosque early, walking to the mosque and not riding [in a car], sitting close to the imam, listening attentively to the khutbah and not fidgeting. Those who manage to do all of these things are very few, as we see from the conduct of people.
Summary of the answer:
The immense bounty mentioned in this hadith is befitting to the immense grace and bounty of Allah, if a person is able to do everything that is mentioned in this hadith and does not omit any of those things. That is easy for Allah, for no one will be doomed except a loser, and whoever thinks that the bounty of Allah is great, no doubt His bounty is even greater than what He thinks.