I have seen concrete used in other steel
boats and the top sealed over with something waterproof - seemed OK after many years. If in doubt, and you were otherwise going to buy the boat, you could easily weld in some steel
plates on top of the concrete to stop any further corrosion
My concern would be that concrete is not very dense so should not be used as ballast in any degree and if it is to be used just as a sealer on top then it was not thought through well. Maybe the ballast is not up to scratch.
Used to be(probably still is) a 53' yacht in Sydney
which looked fine but would knock down and stay down as soon as she poked her nose out of the Heads to go sailing. Changed hands so many times I cannot think. This was a case of poor ballast set in so that the whole keel
structure would have had to be removed to fix the problem.
I am not sure that a surveyor
would pick up these issues. Have to sail the boat to see how tender
she is I reckon.