Being issued from someone doesn't mean he had a claim : a claim is, by definition, something you enforce or want to enforce.
Now, could have he claimed Flanders? Nothing would have technically prevented him, but it would have been a wild move, disrupting the balance he tried to set up in Northern France to prevent an united front against him (the balance break out eventually, but I'm not sure that he would have wanted an earlier Great Revolt).
The rights of House of Metz were quite good, and Henri I had transmitted his own rights to them in order to strengthen their position.
Using his ancestry to grab Flanders wouldn't have make much sense whatever politically (apart pushing a relativly, or in right.