I haven't read it, so I'm afraid I couldn't comment on that particular example.
However, Deathwatch in general are a good example of what I was talking about. Normally Deathwatch marines operate in small groups, like one or two small squads. At that level it wouldn't be unlikely for individual marines to break off and operate independently or with other non-marines.
It's not like SM are averse to working directly with others as a rule, (although some chapters look down on ordinary humans) it's just that they have their own organization and tactics, as well as superhuman endurance, so it just isn't efficient for them to work with IG, sisters, the local PDF, etc. Neither group is trained to work together, and the non-marines won't be able to keep up. Also, the marines are almost assuredly more familiar and experienced with their battle tactics and effectiveness than an outside party, so while they might be willing to follow some general guidelines laid out by an outside party that is in charge, they probably won't bother allowing people to meddle in the specifics of their tactical operations.
But that doesn't really apply if there isn't a whole group of marines waging a campaign together, if it's just a few squads following an inquisitor's orders they wouldn't really be any more effective if they went gallivanting off on their own. Especially so if the Inquisitor is still scoping out the situation, and the Marines don't have a specific target they need to deal with.