Does "exact" determine the precision or the accuracy of something?

When I say "the speed of light is exactly 300000000m/s", is that an incorrect or a correct statement?

That is, does "exactly" refer to the precision of an estimate (then my statement would be correct, I think, because I wouldn't have asserted that my statement doesn't deviate from the correct speed) or does it refer to the accuracy? (then my statement is incorrect, because the speed of light is slightly less than that).

Solution 1:

Exactly in this case refers to the precision of the (presumed) measurement. It does not inherently imply any truth or falsehood about the statement.

If I were to tell you

I have exactly five apples.

it would mean that I want you to believe that I have five apples, no more and no less. But I could make that statement even if I had four or six or 150 apples. You would either have to take my word for the statement's accuracy or submit it to independent verification.

Solution 2:

Raw pedanticism.

It could clearly be either, depending on context.

It seems likely that, in a technical discussion, the term "exactly" implies that the measurement is accurate to the given amount of precision.