[rhdunn]

It's all down to whether or not there is any logic to the guess and check strategy. Most of the computer algorithms work by picking a number in the possible values, following the eliminations, and repeating. If they hit an error, they backtrack to the last decision, choose a new number from the set and repeat.

If you look at some of the Cracking the Cryptic puzzles on YouTube with "computer" in the title, that covers cases where the computer resorts to a guess and check strategy but where better underlying logic is available in the solve path.

[chongli]

The finite nature of sudoku puzzles guarantees that there is an underlying logic to a given move, whether the solver is aware of it or not. Ultimately, all sudoku strategies are backed by an overriding master logic:

"Does this branch of the game tree contain the completed solution or not?"

Whether or not humans have come up with a name for a given strategy depends, almost entirely, on the depth of that branch.

[olliej]

No that’s not the case. Having a single solution does not mean that there is a logical path to it.

If I have a puzzle where I’m down to a bunch of different options and the only way to make progress is to just try fixing different numbers until the puzzle resolved that not a logical progression , it’s an exhaustive search.

[53x15]

If the solution is unique then there is a logical path to it, though maybe not one feasible to discover for an agent with bounded time and (especially for humans) bounded working memory.

Consider the search tree of a backtracking algorithm operating on a SAT-based representation of a puzzle. A leaf of the tree corresponds to some clause that becomes empty after eliminating literals assigned on the path to that node. If you flip the tree upside down and start from the clauses at the leaves, then two leaves with a common parent represent two clauses with complementary literals that can be resolved upon. So any branch of the algorithm's search tree can be viewed as a tree-structured resolution-refutation proof eliminating some value given a set of prior assignments on the path to the branch, and the tree as a whole can be decomposed into an ordered set of proofs for each elimination in the path to an overall solution.

Or, if you prefer, start again from the SAT-based representation and run a prime implicates algorithm like Tison's. Such an algorithm never advances a "guess" or generates a clause that is not entailed. It simply finds one logical consequence after another until it has found all of them, throwing away subsumed clauses along the way until all that's left is the unique solution.

Either way you will always produce a path to a solution and proof that the solution is unique using elementary rules of propositional logic that anyone can recognize. That, I think, is what your parent means.

Of course, the existence of a proof doesn't mean that a human can find that proof, and that's what we usually mean when we say there's no logical solution. That is, nothing new can be proven by scanning the puzzle looking for pattern matches against a fixed library of known proof templates taken to be the universe of logical techniques, and nothing can be proven from less structured heuristic search within the limits of human working memory and patience.