I don’t think this was always the case. I had two separate accounts around 2007ish. Separate passwords for each. <first>.<last>@gmail.com and <first><last>@gmail.com.
I registered my Gmail account in September 2004 (so around 3 months after it was publicly introduced) and the ignore-ruled was already in place then. I'm pretty sure what you describe is not possible.
That's pretty interesting, I just tried it with my aaaaaa@gmail (wrong letters) account.
I sent email to aaaaa.a@gmail and to aaa.aaa@gmail and receive both.
I could be wrong. Its been at least 5 years since I used either of them. I wonder if I had a number or something on one of them.