ARC is not designed to be cheaper or faster to build than ITER. Its purpose is closer to that of DEMO (engineering breakeven). ITER data will be critical for verifying the ARC design.
In the talk here: https://www.youtube.com/watch?v=KkpqA8yG9T4 he talks about ITER being too slow. Certainly the smaller SPARC reactor looks like they want to get things up and running far before ITER is performing fusion experiments, and the possible timescale for ARC is before
While their timescale might be optimistic or wrong, it doesn't sound like they're planning to wait for ITER.
I'm not in the field though so things might not match up with this output or I'm missing something obvious.