If I read a paragraph of text and then try to write it down from memory, I would probably produce something vaguely similar but not quite exact. Maybe I would change some words or phrases with similar ones, or miss a part entirely. The better I understand the underlying idea, the closer it is likely to be to the original, but unless I reread the paragraph many times with the specific goal of memorising it, I'm unlikely to reproduce it exactly.

Same with visualising a bicycle. I've seen many bicycles and I know what parts a bicycle has and roughly how they fit together, but unless I've paid attention to the exact shape and position of each part, I could at best visualise a rough approximation of a bicycle.

I'm not aware exactly what is wrong, just as I would not be aware of what part of the paragraph I changed. But in many ways it doesn't matter, because the high-level idea is there mostly unchanged, just as with the text. The difference is that if you don't fit the parts in exactly the right way, the bicycle will not work, but I'm unlikely to completely change the paragraph by substituting a few synonyms.

This is not even getting into the jump from visualising to drawing, which would depend on my ability to draw.

Right, but accuracy with respect to reality is not the distinction between aphantasia and non-aphantasia, that's more about memory or technical knowledge than about the ability to visualize itself. Someone without aphantasia can also visualize fantastical scenes with no connection to reality, either because it is fully fantastical (dragons and dwarves) or not a real memory (imagining meeting with someone, but it hasn't happened yet for you to be able to recall). They could visualize cartoonish scenes or cel shaded scenes or animated XKCD stick figures. None of those are realistic, but they could still be visualized in detail.