No, you didn't erase anything. Actually you didn't do anything with your SD card.
According to the commands you provided and asuming they all were successful you now should have the contents of the archive in: /and

But there are two things that might have gone wrong:
1. Do you have a mount point called /ant? By default you don't. So if you didn't create it this didn't work. I that case you should have seen an error message here about a missing directory. As a consequence the subsequent cd command didn't work either and the contents of the archive should be directly in / now.

2. Regardless of the outcome of 1. you most likely don't have enough space in / or /ant (you don't have it on an unmodified N900). So you should have received an error message about insufficient disk space.

The worst thing that might have happened is that your / or /home partition is full now.

btw:
There's nothing "magic" about these commands. These are basic unix shell commands. I'd suggest to get familiar with them and understand what they are doing first before you try any further "magic".