This is not what the "Slippery Slope" fallacy means or what was meant by the Domino Theory.

A "Slippery Slope" fallacy (which was never a great fallacy to begin with for lots of reasons, mainly having to do with it constantly being misused) is the pretense that abstract logic demands that some action be taken or not taken because it would inevitably lead to some other consequence; that consequence (or conclusion) is being ghosted as a conclusion for whatever premises are in play, even when those premises themselves will not a) necessarily lead to that conclusion, or b) require the later development of many additional premises that the current problem does not anticipate or demand. In short, the Slope requires that a bunch of other things happen even though they're not actually included in the problem being addressed.

Domino Theory, by comparison, correctly asserted the mutual interdependence of both the various Communist states and non-state actors in the East Asian region. It assumed as a result that attacking any one point of this web of connections would have consequences for the rest of the web - which is basically sound.

Its main error was in assuming, if we may describe it so, that the dominoes involved were all arranged in a circle. They were not. Striking any one domino was not guaranteed to bring down all the rest eventually, or even at all. Laos was not Cambodia was not Vietnam was not China - and certainly was not Russia.

Domino Theory failed because those working under its assumptions did not apparently understand how dominoes work - or at least did not understand (or forgot to understand, or chose not to understand) how they work if they've actually been arranged by a human hand. It did not not fail because it was a "Slippery Slope fallacy."