It is just plain not the case that all interstellar methods are a fortiori terrestrial methods

I'll rephrase it, then: any method planet A can use to attack planet B can also be used by planet A to attack planet A. In the case of an Orion object, some large hyperbolic orbit which intersects the Earth seems possible, but if not, it could always be slowly launched out to whatever distance necessary and turn around.

The Tsar Bomba was deliberately crippled from it's original specifications so as to reduce fallout while still being a sufficiently excessive explosion (although this was only on the order of a twofold reduction).

No doubt that's part of the explanation, but is it the whole explanation? From one of Wikipedia's refs:

Every aspect of the development was rushed. The mathematical analysis normally conducted by the Soviet weapon scientists for a new thermonuclear weapon design was skipped, substituting estimates and approximations of various kinds. This created uncertainties about the system performance that cropped up late in the preparations - leading to eleventh hour doubts, and last minute design modifications even while assembly was underway.

Another part of the ref mentions those modifications were made due to bitter arguments about whether it would work at all. (A quote from a report to the Soviet politicians says the design allowed for bombs of 'unlimited power' - which given the context, is as trustworthy as a $3 bill.)

Reducing the yield and eliminating an entire tamper sounds like something one might do if one is not sure the instabilities have been dealt with. Finally, to quote from one of the rival Soviet bomb-makers:

"At the beginning of 1961 we, who worked in the Urals, had word that our competitors in Arzamas-16 had thought of a new super-bomb. Pretty soon it turned out that it was not some super-discovery, but merely an increase in weight and size. Did that make sense? Building up yields in this simple fashion looked to us both trivial and useless. In those days, we were obsessed with a very different idea - miniaturization, which I have already described. At the same time (and I must honestly confess this) the fuss over the super-bomb idea could not leave us untouched. We were professionally jealous. We looked into the problem and at once spotted two weaknesses in our competitor’s design: their product would be too complicated and too heavy. It could not be squeezed into any of the delivery vehicles - already existing or those still on the drawing boards. Today, I can say quite definitely that we were right. All big bombs followed our way, while the 100-megaton giant - the pride of Arzamas-16 - was made only once - for the test. There was also a replica for the museum."

(Unfortunately he doesn't say whether the complexity and size were solely an issue for delivery or intrinsic to scaling, but it's suggestive.)

Simply taking the calculations used for hypothesizing about Orion weapons and applying them to surface based doomsday device is a gross error. The approach taken and the limiting factors would be entirely different.

Yes, it is a gross error. An Orion vehicle is probably more efficient than a giant nuke or set of nukes, so the lower bounds aren't going to be tight.