I heard on the Dutch news that Trump knew from the start this would not stand a chance, but as the economy is going down and Biden leads the polls, coming up with this is a nice diversion. Also since the elections will likely happen by mail now due to the dangers of COVID-19 really not being under control in November, he would like the safeguard to (ab)use the fact that voting by mail is easier to fraud with, to cause doubt if Biden's victory was actually legit and in accordance to the rules.
Now when I take the words of that reporter into my own logic thinking:
Of course, in a state were over 80% of the votes went to Biden, that trick won't work, but swing states are always the places where the winner and loser are decided, and mostly there the percentages are a lot smaller, he might be able to cause a lot of turmoil about the states where the outcome would be like 51% Biden vs. 49% Trump (yet all electoral college votes, or whatever you call that in English going to Biden).
So the strategy is this. Trump knows this suggestion was futile. Yet it still appears clever. Distract from the problems at hand so the blame he gets for that won't be in the media. If Biden wins (and the polls are really in Biden's favor now with big numbers, but elections have done crazier things, but still) Trump can then say: "See! This is what happens when you don't delay elections to a moment when it's safe to come out and vote, The outcome is easier to fraud with, and it happened folks", hoping that his groupies will cause a situation in which it will be impossible for Biden's administration to function properly.
From populist perspective I must say, this is a cleverly set up trap. All I can advice the American people is.... don't take the bait!