What you without fail, fail to grasp, is the fact that I'm not proving that tBC had more raiders, but that I'm disproving your statement that the percentage of people that cleared the last raid is an indication of the total amount of people that raided actively. But I think I'm catching on to your reasoning.
You haven't seen a tBC raid if your life depended on it and due to this, you have absolutely no clue about the difference between past raiding and current raiding.
Current raiding is linear, while past raiding is progressive, or vertical to make it easier. With that mindset, you could argue that not having cleared the latest instance means you're not a raider. Sadly, or luckily from my pov, that isn't the case with tBC. In Vanilla and tBC, raids progressed like this in terms of difficulty, lowest being the easiest:
[NAXX] for the fans
[AQ40]
[BWL]
[AQ20]
[Molten Core]
[Zul Gurub]
While current raiding looks like this:
[raid with difficulty 2].....[raid with difficulty 3].....[raid with difficulty 4]
[raid with difficulty 1] -> [raid with difficulty 2] -> [raid with difficulty 3]
Every time a new tier is released, the previous tier becomes easier, or more accessible due to gear inflation. So in the current system, I agree, if you didn't finish a raid, you can't call yourself a raider. In the past content however, difficulty scaled without gear inflation, so you had to actually fight for each piece. Any completed raid instance made you a raider and not just the last one.
So once more, your perception of how tht percentage should be interpreted, is off due to your current experience.