rusting

Both statements are correct, but not contradictory. These happen in different stages of rusting.

The process of rusting can be divided into the following stages:
(1) Fe(s) → Fe2+(aq) + 2e- ...(O2(g) + 2H2O(l) + 4e- → 4OH-(aq))
(2) Fe2+(s) → Fe3+(aq) + e- ... (O2(g) + 2H2O(l) + 4e- → 4OH-(aq))
(3) Fe3+(aq) → → Fe2O3•xH2O(s)

Stage (1) is the slowest. In an acidic solution, iron would reacts with the acid to form Fe2+(aq) in a rate much faster than that of stage (1).
Fe(s) + 2H+(aq) → Fe2+(aq) + H2(g)
The overall rate of rusting is thus increased due to the increase in the rate of the slowest stage, stage (1).

However, if the solution is acidic in stage (3), Fe3+(aq) would not formed because the rust is basic which dissolves in the acid. The rust formed can even dissolve in the acid.
Fe2O3•xH2O(s) + 6H+(aq) → 2Fe3+(aq) + (3+x)H2O(l)

If the amount of iron is sufficient, rust would be formed when all the acid is consumed in the reaction between the iron and the acid to form iron(II) ions and hydrogen gas. When this happens, we can observe that the presence of acidic solution would speed up rusting.
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