What's the alternative? Taking the liberty to round it up or down? Both could lead them to trouble. The best would be to mention the stated number with conversion in parenthesis.
I think the latter: 1,000km (621mi) makes it immediately understandable that it’s not 3 digits of precision.
“1,000km (about 600-700 miles)” would feel better.
This is worse because it implies "up to 700 miles" of range, which is not supported by the source.
“Several hundred miles” perhaps?
Nah, that brings the 200-400mi range to my mind, far outside of the 750-1250km range that I intuit is implied by single-significant-digit “1000km” in this context.
Standard procedure in physics is to round to the closest number with the same number of significant digits. In this case, rounding down to 600, so people have a sense of how approximate the number is.