Caution: spoilers ahead for Better Call Saul season 6, episode 7
finale Lalo Salamanca twist is only possible thanks to a huge error of judgement from Mike and Gus. Better Call Saul promised a major cliffhanger ahead of season 6's break, and duly delivered with Lalo Salamanca waltzing into Kim's apartment and executing Howard Hamlin on the spot. Lalo terrorized the couple in Better Call Saul season 5, but Jimmy was assured by Mike Ehrmantraut that their mutual enemy was not long for this world. Jimmy McGill fully believed Lalo died that night, and reacts to his return with the horror of a man who just saw a ghost with a mustache.
Kim Wexler, however, knew better. Earlier in Better Call Saul season 6, Kim noticed a car following her around town and confronted the men inside, assuming they were police. Mike then came forward and revealed the tail was arranged by him as protection. Confirming to Kim that Lalo Salamanca actually survived the assassination attempt, Mike claimed it was "unlikely" the slippery gangster would approach her and Jimmy, but wanted a couple of guys keeping watch regardless. But if Mike had footmen following Jimmy and Kim's every move specifically looking for Lalo Salamanca, how does Better Call Saul's mid-season 6 finale twist even happen?
That question is addressed elsewhere in Better Call Saul's "Plan & Execution" episode, and the answer is part stroke of genius from Lalo, part calamitous blunder by Mike and Gus. When "Plan & Execution" first catches up with Lalo, he's staking out Gus Fring's superlab site seeking a route into the heavily guarded building where proof of Gus' cartel betrayal surely awaits. From his sewer hiding place, Lalo tries phoning Hector, but when the line crackles, he realizes Gus Fring tapped the phones at Casa Tranquila. Instead of telling Hector the truth ("I'm outside Fring's secret construction project and nearly have the proof we need to take him down") Lalo lies to his uncle, claiming his investigations hit a dead end, and the original plan to assassinate the Chicken Man is back on. This isn't true, of course - Lalo is intentionally misleading Gus.
The ploy works like a charm. A panicked Mike approaches Gus at a publicity event for Los Pollos Hermanos, and plays his boss a tape of Lalo's phone call to Hector. The recorded conversation convinces Mike that Lalo will be making an attempt on Gus Fring's life imminently, and explains how he relocated guards on low-level targets to converge upon Gus' house. During his conversation with Kim in "Hit & Run," Mike described her and Jimmy as unlikely targets, so we safely assume any henchmen following them are moved to Gus Fring's home following Lalo's phone call. A lack of guards allows Better Call Saul's tricksy Salamanca to finally enter the lawyers' apartment. Lalo probably knew Jimmy and Kim would be protected - hence why he hasn't visited already - and probably also knew they'd be left unguarded after the phone call to Hector.
Mike & Gus' big mistake is not considering the possibility Lalo played them. Not once did either man ponder whether an experienced criminal like Lalo might know when his phone line is compromised; nor were they suspicious at how their wily enemy suddenly gave up finding proof. Depending on what Lalo asks Jimmy and Kim to do, that mistake might've just sealed the Salamancas' victory (if you forget about Breaking Bad for a moment...)
Mike absolutely 100% believes the tapped Lalo phone call is reliable intelligence, and that's an error he'll no doubt learn from ahead of Breaking Bad. But while Gus Fring outwardly goes along with Mike, it's possible the boss actually sees through Lalo's deception. Previously, in Better Call Saul season 6's "Black & Blue," Gus experienced an epiphany over spice curls that made him realize Lalo was secretly tracking down the superlab. Away from Mike's view, Gus then secretly hid a gun at the site, seemingly expecting this would be the arena where he and Lalo finally settle their business. Since Gus didn't share this revelation with Mike, and since he deliberately hid the gun without his right-hand man seeing, maybe Gus does realize the phone call was bogus, and does know Lalo is still heading for the superlab, but plays along with Mike to ensure the clandestine showdown he was preparing several episode ago isn't interfered with.