Doesn't make much sense to keep a modification to prevent escalation with russia a secret - we want ukraine to know so they don't make any plans that rely on capabilities they don't actually have, we want Russia to know so that they don't think they need to pre-empt a strike that isn't coming, and we want everyone else to know that we're not escalating the conflict. The only reason I can think of to keep the modifications a secret would be to bluff that they actually did have the full range, but in that case why not just leave the systems unmodified?
Because they're pretending to help Ukraine while keeping them from being able to do anything decisive while the war grinds on in trench-warfare-on-the-western-front style.
66 billion dollars is hardly pretending
[Edit] Less snark
The smart route is to do all of that, but still modify them to remove the possibilty of a rogue person or unit from doing it as well. Its called covering all bases. When the stakes are this high (nukes), you ensure there are no loose ends.