A max-width was set at both 768 and 1168 pixels in commits related to PR 559. Early versions of the cover block had hardcoded H2 headings, but that might have changed even before Twenty Nineteen and WordPress 5.0 were released.
If the max-width is not necessary for any case with these headings, I would prefer simply removing the h2 selectors from those styles instead of unsetting later.
Otherwise, we could override it when users specify any text alignment. Left and right alignment do not work well either (when they go against the language direction).
.entry .entry-content .wp-block-cover.alignfull h2[class*="has-text-align-"] {
max-width: unset;
}