Usage of "could" in past to indicate "statements of fact"

Question

I am aware of the usage of COULD in the following cases (list from Cambridge Grammar (https://dictionary.cambridge.org/grammar/british-grammar/modals-and-modality/could)):

• possibility in the present and the future.
• suggestions and permissions
• past ability
• past single events with verbs of senses and mental processes

Also, this makes clear that sentences like "I could visit you yesterday but I was lazy" or "He could travel to China a week ago" are incorrect.

But then the are sentences like:

• Hundreds of years ago, even a simple injury could be fatal.
• In the ancient Rome, you could have both male and female lovers and nobody wondered.

For the following examples, I was told that these are statements of fact:

• I could buy it yesterday and I did.
• Back then, travelling could be dangerous.

What usage of "could"would that be? I cannot see a fitting case in the overview. Why those are correct even when "COULD" refer to the past events?

Answer 1

A simple injury could be fatal is past possibility or past capability. Not all simple injuries were fatal, but one could be. It is not a simple statement of fact that all injuries, even simple ones, were fatal hundreds of years ago. If you had a simple injury it might prove fatal. It was capable of killing you.

you could have both male and female lovers again refers to a past possibility or a past capability. Gender was not an irrelevancy in all love relationships in ancient Rome, but it was possibly an irrelevancy. Gender could be irrelevant as far as people were concerned. A person was capable (not prevented by societal taboo) of having lovers of either gender.