A rule book doesn't trump the law, regardless of any 'catch all' clause.
Hearts will almost certainly be arguing there's a disconnect between the rule book and the law in their particular situation (as opposed to arguing the rule book wasn't properly followed). In what regards we can only guess.
Nick de Marco QC recently wrote a piece in the Sports Law Bulletin about the implications of the failed South Shields appeal. His final concluding paragraph indicates areas where such disconnects may occur:
"The case is not, however, a carte blanche to governing bodies in these uncertain times. The FA, and other regulators in sport, remain bound by ordinary public law and contractual principles. Irrational decisions will continue to be challenged; and procedural fairness must be achieved notwithstanding the unprecedented times in which we live. The ability to amend or vary rules due to COVID-19 must be exercised in accordance with the rules or articles of association of each relevant league. Where decisions are made collectively by votes of member clubs (as is the case in levels above those considered in this case) unfair prejudice and/or competition law principles may also come into play."
The full article is at: https://www.sportslawbulletin.org/football-time-covid-19-lessons-be-learned-recent-decision-south-shields-fc-v-fa/
thanks to Paulh66 at Non League Matters for this.