That scene was created for the trailer. It's the same shot as the scene of Jyn heading for the tower console used in the film, but it didn't seem dramatic enough out of context so they dropped a TIE in there for the trailer.
As I understand the scene was in fact shot for the film, it would have been followed by the X-Wing fleet arriving and shooting it away, allowing Jyn to proceed, at the same time the X-Wings and U-Wings taking aout the AT-Walkers, particularly the barrage against the AT-Walkers, and shot of the being taken out.
According to the editor, there was a redundancy of shots and pacing issues, so at least one scene (among many other we didn't even see) had to go.
The trailer editors meantime, just choose from what's available, regardless if it will make the final cut.
As far as why it was a standard TIE and not a Striker, could be to parallel it against the classic X-Wing which shot it out, or for the impact of hovering there, and proximity to Jyn the Standard TIE wing configuration allowed for a stronger composition.