"Even just a hint" doesn't deserve to be squirreled away in a mere paranthetical. It's what distinguishes some bullshit (the article's suggestion, whereby the allocator carefully checks that you supplied the right value that it already knows and barfs if not) from the actually potentially useful improvement to the API that you suggest.
The code that frees very often does not know how much was allocated.
An allocator that fully trusts the size provided to `free` is hugely unsafe when the size turns out to be incorrect. As an extreme example, the allocator may know that every 4-byte allocation is put into the arena that is aligned to 0x1000-byte block, so it may jump straight into the arena header which won't be there if the size was incorrect. So a real-life allocator is likely to do some checking even when the size is provided, but that checking can be much more simplified in that case.
The code that frees very often does not know how much was allocated.