This is not judgment at all, and has nothing to do with the Ready for play. Defense substituted 2 for two. The two replaced players didn't get off the field before the snap. That's as objective as it can get.
Allow me to try a different approach, On the top of page 6 of the NFHS 2020/21 "Game Official's Manual, it suggests, "Game officials must have a football sense which supersedes the tactical application of the rules so that the game goes smoothly.
Game officials are expected to exercise good judgment in applying the rules."Such judgment is NOT an excuse to ignore the rules, but provides some discretion in applying them. Football is a game of seizing advantages, but only when those advantages are earned and appropriate for the situation involved. Being prepared to substitute to gain an advantage by a substitution is usually an earned advantage, however if it is executed in such a way to deliberately and inappropriately dis-advantage the opponent, it may create an unfair dis-advantage. The "official's judgement" in applying the strict letter of the governing rule may, in fact, may actually support the unfair effort by holding the opponent's reaction to a strict tactical interpretation, whereas a slight judgmental adjustment, to the circumstance, may actually better allow the stated purpose of the rule to balance and prohibit any unfair advantage being suffered by either contestant.
As the above approach suggests, such judgments that negate inappropriate efforts, without creating any unfair advantage being gained by the opponent, allowing the situation to continue as basically intended, assists "the game to go smoothly".
Page 6, further advises, "Preventive officiating" facilitates the smooth conduct of a game within the rules.
The good game official, by actions and/or by words can frequently prevent certain fouls from happening." These suggestions relate specifically to High School level rules, ONLY. Higher level contests and designed rules, considering the more extensive maturity and experience of players, may prefer stricter technical compliance.