To be fair we are both correct.
Physically the first 512 bytes of partition sda1 is the MBR. This is a 100% bomb proof statement if you run only one system too and it is in sda1.
However if you run multi operating system and to boot them your MBR has to form a linkage to the operating system that carries out the multi-booting function for you. Therefore the MBR is taken for the whole disk.
As an example, say you have XP in sda1 with boot loader NTLDR, Slackware in sda2 using Lilo and PCLinuxOS in sda3 using Grub. Each bootloader has Part A (the first 512 bytes) and Part B (the part that actually boots the system). A boot loader reside in the boot sector which is always reserved regardless if it is used or not but a few filing systems the break the rule.
If you let XP boot the rest then Part A of NTLDR will assume the MBR position.
For PCLiniux's Grub to multi boot Part A of Grub must be installed in the MBR even though its master resides in sda3. Basically the Part A (the MBR position) of sda1 is overwritten by the Part A from sda3.
Similar story for Slackware if Lilo were to boot all three system.
Generally if you talk about the MBR in a multi-system environment you have to be talking about the system that controls the booting for you and it is rare but possible that it is "sourced" in sda1.
To prove if the above is a load of rubbish
Next time you boot up a Linux with Grub, say PCLinuxOS, press Grub to get a Grub prompt and type these two lines
You will find Grub report the first as using the "whole" disk, as (hd0) means sda in your case.
and response to the second statement would be "filing system unknown" as your sda1 is empty.
The fact that you can operate booting successfully with an empty sda1 is an indication that you are using the MBR for the whole of the hard disk and not for sda1.
The other angle that may help you to understand the MBR is if you have several hard disks then theoretically you have many MBRs but only the one at the front of the booting queue matters. You can let disk sdc be the first booting disk and then the first sector of 512 bytes of sdc1 partition will control your booting and that MBR can be "sorced" from any disk you choose.
The third point that may be helpful to the appreciation is you can delete sda1 completely without affecting your current system. If you haven't got sda1 but the system still boots then the MBR must not be affected by the existence of sda1.
Hope the about is clear. You are right in bringing it up as it is confusing and many Linux users wouldn't have a clue about the difference. Don't think I know that much about it either but the above is my "obervation" and "experienece" after booting 145 systems in the box.