Does my computer get securely erased if I do it through Find my iPhone?
The erase is very secure - I've not heard of a single success in recovering any data from a one pass zero with modern hard drives. As to what really happened, it depends on what setup your computer has.

In the case of FileVault 2, the erase process is very rapid. Everything on the drive is scrambled - even blank space looks no different than a file like a spreadsheet or a picture. The erase command simply destroys the cryptographic keys and effectively destroys everything in a short amount of time.

In the case where you don't have Filevauly, I believe a normal one pass zero is performed based on how long it takes. I don't see any official documentation from Apple and also haven't seen someone forensically analyze an erase (or trap the filesystem using a remote debugger or a SATA drive that logs commands) to be sure.

Either way, at the end of the erase process, you get an email, so when you send an erase, you'll know when it started and when it finished and can conclude which erase actually happened.

In your case, your Mac probably received the erase command nearly instantly, so you can now perform a single pass zero (assuming you aren't going to send in the drive for data recovery) and time that to compare with Apple's erase. A seven pass erase is basically seven times as long as a one pass erase so the timings should be easily observable to deduce the type and number of passes.