Justice League Snyder Cut will not be on widescreen and even when you try to adjust the settings of the film on your television, it would just either be cropped in or zoomed-in so do not be surprised when you see that the film isn't stretched to the ends of your screen – it is intended to be that way.
Zack Snyder's Justice League is the original director's cut of the film – the version of the director before he was replaced by someone else as he left halfway through the production of the film as the production company and the director did not share the same creative vision for the film and caused the creative difference for a change in directorship with Joss Whedon.
After years of calling for #ReleaseTheSnyderCut, the fans are now in a victory that the film has now been given the chance to be presented. Although Zack Snyder's Justice League is not canon to the DCEU as personally acknowledged by Snyder himself that the Joss Whedon version of the film is the one canon, having the director's version of the film see the light of day is already much to the delight of the fans.
And no, it is not the same as the theatrical version. It is an entirely different film with a lot of notable differences that it is almost a completely different film with only 10% to 15% of it included in the theatrical release back in 2017.
As released in the trailers, it's no surprise that even on the uploaded teasers of Zack Snyder's Justice League, the film seems to be cut in a square instead of a widescreen frame. The film will definitely have the same cut as well. It is intentional and part of the aesthetic that Snyder was going for.
First, what is an aspect ratio? It is the ratio between the width and the height of the frame of the picture on the screen. Widescreens usually have an aspect ratio of 16:9 which has variations of either normal widescreen (1.85:1) or anamorphic (2.39:1) depending on the director or the director of photography's choice. It is the safe choice for the DVD release as well as it is the DVD standard.
Joss Whedon's Justice League is in the 16:9 or 1.85:1 aspect ratio (it's almost the same, just removing the decimal) which is the common cinema standard of the US widescreen, one which has the top and bottom cut off.
In Zack Snyder's version, however, it is in the 4:3 or 1.33:1 aspect ratio. In comparison to the Whedon version, its cut is different as it has a semblance to a square cut. This is the aspect ratio of the earliest films and it is usually used today for aesthetic purposes.
If you have watched the series of WandaVision, you'll be able to see both of these aspect ratios in one go. Given that WandaVision started with the sitcom style of storytelling, the sitcoms are in the 4:3 aspect ratio, reminiscent of the shows that were released in the era portrayed. But when the scene shows what is happening outside of the sitcom, the aspect ratio changes to 16:9.
Justice League Snyder Cut will have a 4:3 aspect ratio all throughout. It is the personal choice of Zack Snyder himself to set the film in such an aspect ratio that stemmed from the full-frame render of the IMAX scenes that he shot in Batman v Superman: Dawn of Justice. Since then, Snyder had the idea of using the same aspect ratio for his future films.
In the July 2020 Justice Con, Snyder revealed that his version of Justice League is intended to be shot with the said 4:3 aspect ratio in his mind so that the film could fill up the 1.43:1 IMAX screen. It will not only be for select scenes, it would be for the totality of the film, not like the IMAX versions of some films that only have that aspect ratio for selected action scenes.
Snyder didn't use IMAX cameras though, but he shot it with a ratio suitable for the 1.43:1 ratio ideal for theaters. Snyder shared the reason why he intended the said aspect ratio of the film saying, "Superheroes tend to be, as figures, they tend to be less horizontal. Maybe Superman when he's flying. But when he's standing, he's more of a vertical. Everything is composed and shot that way, and a lot of the restoration is sort of trying to put that back. Put these big squares back. ... It's a completely different aesthetic. It's just got a different quality and one that is unusual. No one's doing that."
This means that wherever you see the film, the bars would be on the sides, not on the top and bottom like that in widescreen, just as how Snyder wanted it to be.