"trends from 400M visitors" vs "trends by 400M visitors"

I have a problem with [...] the preposition [...] "from"

How the pandemic changed traffic trends from 400M visitors across 172 Stack Exchange sites.

I tend to agree with you. I had to read it twice to see what the author intended, and I too expected "to 500M...".

The preposition “from” is basically concerned with the locative origins of something:

I went from France to Italy. My original location was France…

I’m from France My original location is France…

It was made from gold mined in this valley. The material used to make it was gold that originated in mines in this valley

You can’t get blood from a stone Blood cannot originate in a stone

I caught the disease from a cow. A cow was the origin of my disease

The prepositional phrase in question modifies “traffic trends”

How the pandemic changed traffic trends from 400M visitors across 172 sites = How the pandemic changed traffic trends that originated in the 400M visitors across 172 sites(?) - This suggests that the traffic trends had their origins in 400M visitors whereas it is the change in the traffic trends that is the real subject and the pandemic that is the cause.

The immediate idiomatic alternatives are “by” which, as you point out is adverbially instrumental, and “of” which creates a contextual adjectival association.

How the pandemic changed traffic trends by 400M visitors across 172 sites = How the pandemic changed traffic trends by the use of 400M visitors across 172 sites– here there is an implied “an increased 400M visitors”

How the pandemic changed traffic trends of 400M visitors across 172 Stack – this implies that there are regularly 400M visitors (traffic) and they have trends and the trends have changed. I assume that this is what was meant.