It’s just optics and formalities. To those who are concerned, read the actual letter. The letter explicitly states that this doesn’t decide that Biden is the winner, but only that since Biden is currently the “apparent” winner and with all the lawsuits pending, the Biden team has the right to access post-election resources for security reasons. However, this can be simply overturned if Trump is declared the winner. Along with this, Trump’s tweet also helps him get rid of that “coup and dictator” image that the media is presenting because he hasn’t conceded.

The election will be officially over when the all the lawsuits are withdrawn and Trump has officially conceded.