I'm pretty sure that's exactly what it does (or claims to at least). See here: https://www.interactivebrokers.com/en/software/tws/usersguidebook/algos/combo_orders.htm
See where it says that once "the spread becomes marketable " it submits a limit order then a market order... pretty sure marketable means that only when the spread is at the spread price you want to buy at or lower does it submit the orders. this is not the same as submitting a limit order before the spread reaches your desired spread price. the latter is much better because it is displaying and quoting size in the hopes that someone will take market and hit your limit order bid. this means you get a better price than if you took market on the spread itself. sometimes IB wont even send the order out until its damn cetain it will get the spread price meaning if your spread price you wish to buy at is exactly 1.00 the offer of the spread must be either 0.99 sometimes before it even sends the order out. If you quoted a limit order first ( like an autospreader does) you could possibly get the spread at 1.00 when the spread is offer price is at 1.01... because someone hit your bid. A small difference but makes a big difference overtime.
How could unchecking " Non-Guaranteed" be anything but simply putting in a simple limit order for the spread? There is no way IB would assume the risk unless they have an edge over you. Fishy. This has to be stocks only and not futures.
I still think even if you select non-guaranteed it will still only send the first limit order out as soon as the spread price is marketable. Marketable meaning you'll get filled right away similar to taking a market order.
One last try...the word marketable isn't anywhere to be found and it purports to do exactly what you want. REL + LMT, where one or both of the legs are submitted as a simulated relative order (at the bid for a buy and at the ask for a sell). If only one leg fills, the second leg is resubmitted as a limit order. REL + MKT,where one or both of the legs are submitted as a simulated relative order (at the bid for a buy and at the ask for a sell). If only one leg fills, the second leg is resubmitted as a market order.
What if the second leg price runs away from you? It resubmits the LMT at different prices? Obviously, the desired price will not be achieved.
Well, that's the choice you have to make between REL+MKT and REL+LMT. All spreaders face the same issue.