It looks like the building bending,it could be a steel beam.But we can't know for certain.Gotta wait for the anime.Also,even if there is metal there,we have no proof that Kidd can't use assign into non living things still.All awakenings so far have affected the enviroment.
That's the point we have proof as of right now.

If what you were suggesting were remotely true he could have contructed his weaponry with thus not metal. He has never once done so. Nothing suggests he can. There is nothing on panel that shows it. So it's headcanon to assume something we have not seen.

You may be 100 percent correct. However it is yet to be seen
 
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