The Chronurgy Wizard's level 14 ability says

When you or a creature you can see within 60 feet of you makes an attack roll, an ability check, or a saving throw, you can use your reaction to ignore the die roll and decide whether the number rolled is the minimum needed to succeed or one less than that number (your choice).

Am I correct in interpreting this to cause someone to succeed or fail even if it should be impossible? For example, someone's got to roll a DC 10 Constitution saving throw for concentration, but they have a 13 to Con saves. Can the Chronurgy Wizard cause their roll to magically be a -4, making them fail?

If Chronurgy wizards can do this, how does this apply to ability checks? It's suggested that a DM only roll for an Ability check if it's actually possible for the players. For example, if you have a DC30 ability check, but the player attempting to pass it only has a 6 to the relevant ability, you don't roll and just say they fail no matter what. But if you have a Chronurgy Wizard at the table, you should let them try anyway, and if the Chronurgy Wizard wants them to succeed, they succeed (to avoid metagaming, you should make an actual dice roll and not tell anyone that this is impossible without Chronurgy shenanigans). This implies that any event in the game that could feasibly be done with an ability check of any type, regardless of the DC, can be accomplished with the help of a friendly Chronurgy Wizard.