Given your clarification of the people you say see Communism/AnarchoCommunism as non-reactionary being "the public": They do. Not all obviously, but especially in the west. Reactionary opinions and stances on policy and social issues is generally associated with the right, and for good reason.

Reactionism is only reactionary from the view of progress: If the reaction opposes the progress it is "Reactionary", and not just a reaction.

Historically, the left (including the economic philosophy of Kropotkin, Marx, Engels, etc.) is generally on the progress side.  (Hence "Progressive" being negatively associated with the left and communism.)

Thus your hypothesis is at least noticing the reason: Communism does indeed stress the forward march of progress, and so when a NEW policy is put in place, it is generally REACTED to, whereas the abolition of OLD policies could be thought of as generally being related to progress.

This is obviously not always true - some old policies are still progressive, and are being removed even now, and by people on the right at that - and my answer is not necessarily based on pure fact, but also on anecdotes, to feel free to correct me as you see fit.

Given your clarification of the people you say see Communism/AnarchoCommunism as non-reactionary being "the public": They do. Not all obviously, but especially in the west. Reactionary opinions and stances on policy and social issues is generally associated with the right, and for good reason.
Oxford Languages Defines Reactionary: opposing political or social progress or reform

Reactionism is only reactionary from the view of progress: If the reaction opposes the progress it is "Reactionary", and not just a reaction.

Historically, the left (including the economic philosophy of Kropotkin, Marx, Engels, etc.) is generally on the progress side. 
(Hence "Progressive" being negatively associated with the left and communism.)
Wikipedia: In the United States, communism is widely used as a pejorative term as part of a Red Scare, much like socialism, and mainly in reference to authoritarian socialism and Communist states.

Thus your hypothesis is at least noticing the reason: The left does indeed stress the forward march of progress, and so when a NEW policy is put in place, it is generally REACTED to, whereas the abolition of OLD policies could be thought of as generally being related to progress.

This is obviously not always true - some old policies are still progressive, and are being removed even now, and by people on the right at that - and my answer is not necessarily based on pure fact, but also on anecdotes, to feel free to correct me as you see fit.