If the Coastal Commission approved, and the terms of its permit are met, it's not against the (human, at least) law.

As far as Nature is concerned, as with all coastal protection (especially "bulkhead" style structures as seen in the rendering in the Coast News article), it's temporary, and will probably not last until the stated 2050 EOL under any circumstances.

As for the "ultimate solution" proposed by the "civil engineer" -- yes, that will eventually happen whether or not the seawall is built. Once the railroad is in a tunnel inland, and is abandoned along the cliffs, the owners at the top of the hill will need to arrange for protection works (if any are feasible once the bulkhead is ripped out as required, apparently, by its coastal permit) on their own (or, more likely, some public agency or agencies' given the political clout of clifftop property owners) dime. I'd bet on the last. If/when that public help is not forthcoming, the blufftop property owners will have to make some run-or-fight decisions. Fighting will not be effective once the bluff erodes enough (supplying sand to the beach, at least) to remove the remnants of the railroad shelf.