"designed to be attached" is different than "capable of being attached"..........a plastic 2ltr bottle is capable of being attached, but isn't a silencer until it is attached.
No disagreement there, which is why we understand that one is a silencer by itself, the other only if used as such in much the same way that a weapon is a weapon, but a tool or other implement that is not ordinarily a weapon becomes a weapon under the law if used as one. The corollary, though, is that something which
cannot be attached to a portable firearm obviously wasn't designed to be attached, and if it cannot be, then it is not a silencer. A stack of tires or a 55 gallon barrel with a hole through both ends was not designed to be attached, nor can it be (at least in any way that would leave the firearm a portable firearm, which
is part of the definition of silencer), ergo ATF's determinations that such things are not silencers. The language in the very determination letter you are citing to support your argument is specifically referencing "attached" and "portable" as defining characteristics, "attached" more than once. That's about as clear as it gets with ATF.
And for the record, simply affixing a bottle to the muzzle of a firearm does not constitute the making of a silencer. It could certainly be considered a solvent trap, which is not a regulated item. It doesn't become a silencer until it is
"....... redesigned, and intended for the use in assembling or fabricating a firearm silencer or firearm muffler", which would, at minimum, mean having a hole out the other end in line with the bore.