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Two marathons will never be run in the same conditions, that is the nature of outdoor sports.

Besides weather, there are loads of factors in the performance: shoes, clothes, food, etc. So basically every record gets an asterisk?

No asterisk needed. The criteria for record-eligible courses have been clearly defined. The weather was good, but not quite ideal. In slightly colder conditions I think Sawe could have gone a few seconds faster.

Unless there was a 2ms+ tailwind on a one-way course there is no asterisk needed. All outdoor running is done in variable conditions.

So if the weather was bad the accomplishment would mean more then? I don’t think this is how it works. Sports don’t happen in a vacuum.

I am impressed by your ability to delineate the weather effect on his run with such confidence! Particularly given advances in other variables that contribute.

Better weather has, to the best of my knowledge, never been part of marathon record keeping. People do note in accounts of (e.g.) the Boston marathon that the weather was particularly atrocious in some years (hence a general slow down across the field), but weather "aided" fast times are not considered illegitimate or even worthy of note.

Obviously, barring wind, which is why some marathon courses are not eligible for world records.

That’s a wild reason to withhold a true record. People run marathons in all sorts of conditions since it became a thing. It is unlikely this is the best weather ever for a record set and even if it was, it’s never been a factor when deciding to qualify a record. That’s beyond unfair.