Rabastan had not assumed anything. What he guessed about his guest was based on the circumstances and what he had observed about the man since being in his presence. His educated guess, then, told him that it was highly likely this man was a servant of the Dark Lord; considering the fact that he'd just made his bid for the Dark Lord's attention on Friday, it seemed the most likely explanation for how he'd caught the older man's attention. Nevertheless, there were other possibilities: he could have noticed Rabastan on the trip to Kenya with Mr. Smith, or heard about him from one of Mr. Smith's colleagues. Or he could be somewhat else entirely.
Whatever he was, he was older and more powerful, and Rabastan thought it best to assume that the man held a higher rank than he. Pride he had in spades - his family was relatively new to England, but they were old French wizarding blood and old money, much of their fortune invested heavily in well-respected French vineyards. Their wine was wasted on one who liked vodka, but liquor and other types of food and drink were to be had in plenty in the Lestrange household, served by a loyal elf. And he had treated his guest according to the status he believed the man held, regardless of which hierarchy he held it in.
It surprised some, but acceptance of authorities higher than himself came relatively easily to Rabastan. From his seat, setting down his own glass of vodka (for he would not insult his guest by having something else, especially when he liked the strong drink) he smiled and spread his hands in a gesture of openness and honesty. Precisely how honest he would actually be, he'd yet to decide, but it was always best to start out on the right foot.
"It was not my intention to confuse you," he assured him. "Perhaps if you tell me what you find confusing, I can explain myself."