وَجَحَدُوا بِهَا
...
And they belied them,
means, verbally,
...
وَاسْتَيْقَنَتْهَا أَنفُسُهُمْ
...
though they themselves were convinced thereof.
means, they knew deep down that this was truth from Allah, but they denied it and were stubborn and arrogant.
...
ظُلْمًا وَعُلُوًّا
...
wrongfully and arrogantly,
means, wronging themselves because this was the despicable manner to which they were accustomed, and they were arrogant because they were too proud to follow the truth.
Allah said:
...
فَانظُرْ كَيْفَ كَانَ عَاقِبَةُ الْمُفْسِدِينَ
So, see what was the end of the mischief-makers.
meaning, `see, O Muhammad, what were the consequences of their actions when Allah destroyed them and drowned every last one of them in a single morning.'
The point of this story is: beware, `O you who disbelieve in Muhammad and deny the Message that he has brought from his Lord, lest the same thing that befell them befall you also.'
But what is worse, is that Muhammad is nobler and greater than Musa, and his proof is stronger than that of Musa, for the signs that Allah has given him are combined with his presence and his character, in addition to the fact that previous Prophets foretold his coming and took a covenant from the people that they would follow him if they should see him, may the best of blessings and peace from his Lord be upon him.