There are many modes and canons of interpretation a lawyer might consider.
Among these, consider the whole-text canon and the presumption of consistent usage within a text, especially for a public-law statute.
If different sections within a statute (or in rules interpreting or implementing different portions of a statute) use a common word and use it, at least partially, without reference to a special definition, one might presume the word ought to have some logically consistent meaning across the whole of the statute.
If C’s shares in corporation X had been C’s capital asset, does anything in the Internal Revenue Code of 1986 (unofficially compiled as title 26 of the United States Code), or a rule or regulation under it, require C somehow in her tax return to report or treat the redemption as a disposition of the capital asset? Does C have a capital gain, or a capital loss?
If X was (in 2020) a subchapter S corporation, did anything in its tax-information reporting show the change in shareholders?
If X is (in 2021) a subchapter S corporation, how will it apportion the income passed through to the remaining shareholders—to A and B 50/50?
If an unrelated purchaser buys X (not as purchases of assets from X, but rather as a purchase of all X corporation shares), would A and B (but not C) get the price the purchaser pays for the shares?
Following out possible consequences of X’s redemption of C’s shares might show that, whatever the legal form, the redemption might have been, economically, C’s disposition (and perhaps, in some senses, A’s and B’s acquisitions).
A presumption of consistent usage might be persuasive if other aspects about possible interpretations are in equipoise (or if this presumption outweighs other aids to interpretation).