There’s a big difference between having a well established population where food is plentiful and predators are (relatively) scarce.
A transitional form which may be the first of it’s kind or one of only a few, doesn’t have the luxury of being able to get by like seals do.
None of which detracts from the point that this intermediate limb of a seal is both a functional leg and a flipper and a seal survives just fine without it being fully a leg or fully a flipper.
A “transitional form” may be a species that numbers in the thousands and exists for hundreds of thousands of years. For example the Australopithocine existed for millions of years over a wide range, yet people point to it as a “transitional” form between knuckle walking apes and bipedal humans.