My reason for thinking that it is Lurantis comes down to a few things. The Z-move slot being taken up by Z-Heal Bell is an indicator that it probably isn't occupied by Virizion, which generally speaking doesn't tend to run non-Z sets off of Veil squads. The team's inability to consistently switch into Rhydon tells me that it is unlikely to be something as frail as Sceptile, and as such this narrows it down to four potential candidates for me in Lurantis, Whimsicott, Scarf Mesprit, and Rotom-C.
While Scarf Rotom-C being taken is a big part of why I didn't choose it (this is definitely very possible due to Typhlosion massively appreciating pivots (and due to how Scarf Rotom-C patches up this team's lack of speed control to an extent)), I feel that, because the aim is to keep Typhlosion as healthy as possible throughout the match, Xatu on its own simply isn't adequate hazard control for a team like this given that it loses to Piloswine, loses to Rhydon, and can be put under a large amount of pressure by more offensively-invested Heavy Slam variants of Steelix. While Scarf Mesprit can patch this up to an extent with a fast HWish, I feel that it is more likely that some kind of hazard removal is present here so that Typh isn't restricted to Fire Blast if SR gets up through Xatu.
Lurantis is more appropriate than Golbat in the role because it actually beats the common SR setters (Rhydon, Steelix, Piloswine has to play a game with Leaf Storm vs Defog when switching in), whereas Garbodor is going to have a very hard time getting hazards up against Xatu outside of shit like Poisonium Gunk Shot, meaning that being able to clear it ('cause Golbat doesn't Defog vs the latter anyway) isn't as important. It is more appropriate than Hitmonlee IMO because, with it as the choice of secondary hazard control, Rhydon can come out in front of any of Xatu, STAB-locked Cinccino, or STAB-locked Typh and click one of its STABs without any drawbacks.