Help needed... Sanded through the veneer 🙁  

I worked in a furniture refinishing shop for about 10 minutes in college. They employed a lady who was fantastic with a brush and paint. She painted wood grain to perfection. You could try that if it does work it is easy to remove.
Re-veneering the entire top is also an option.

It happens to everybody at some point. If you don't have the tools or skills to make a good veneer patch, take it to someone who does. It's a straightforward job and shouldn't be prohibitively expensive. Yes, *you* will always be able to find it, no matter how well it's been repaired, but others may not. Besides, an honest repair is preferable to what you've got now, IMO.

Thank you both for your messages. I thought the top was solid wood and used a sander. If I had realized it was veneer, I would have never done that.
Such a pity as it looks fantastic now that it is refinished.
I guess I will take it to a pro.
Thanks again!

Both options are good!
Sorry I haven't responded sooner, but Leif's suggestion offers an acceptable choice. The success of either approach rests entirely with the skills of the restorer. I guess my decision might depend as much on anticipated usage. Or I might just leave it to the next owner...

As a precaution to others, one can usually determine whether a panel is solid or veneered by comparing the two faces, front and back (or top and bottom). In solid work the seam locations and grain appearance will match; in veneer, not. How edges are handled is another source of information . . .
Would experts here be able to confirm, or deny, that one very seldom finds solid-stock panels in which the grains are book-matched or otherwise closely matched, as is common in veneered panels ?

It certainly is unusual to find solid wood piece with book matched, or otherwise closely matched grain. Simple logic will suffice in lieu of an expert: grain changes too much when cut much thicker than veneer. Therefore, except in limited cases, say where the boards are extremely wide, it just is not possible to get a close match except with veneer.
For example I have a solid teak dresser with boards that are "grain matched", probably by accident, but it is hard to see this fact because it is no longer a close match cut with the planes cut that far apart.

You certainly could have done a lot worse there, if that's the extent of the damage. Leif's suggestion is good. People use oil paints and some use gel stains. Personally, with that thin a line, I would probably try a gel stain, lightly, and see what happens. Not sure the color of the Teak with a finish on it, but I've had a lot of success with Old Masters 'Cedar' when it comes to getting that aged, reddish hue.