Garp and Sengoku were enrolled into the marines the same year as this happened. There's indication they were already part of the Navy by the time this happened, much less that they were of a rank high enough to even step foot on Marijoa.
The threat posed by Xebec, seemingly just another royal figure at that point, was first needed to be assesed by the admiral before a "I don't give a crap about CDs, this mf needs to be obliterated at all costs" decision could be made. This "assessment" more likely than not cost the admiral quite a bit as the fight started. Again, I'm not saying the admiral fought the whole fight holding back, or that even if said admiral went out from the start they'd win. It's just an important factor to consider.