This really means nothing. There are some galaxies that look like modern ellipticals at high redshifts but there ages are all young. When their ages are measured they are consistent with a Universe that is 13.7 Gyr old. So for example we can measure the ages of galaxies about 9Gyr ago and they alway turn out to have ages consistent with being 5Gyr or younger at that point (thanks to some very important work by Maraston et al. on the effects of TP-AGB stars on stellar spectra).

Whats more these galaxies are mostly less massive then the equivalent galaxies in the nearby Universe.

Furthermore the ages of the Globular Clusters (which mostly formed during the very earliest epoch of star formation i.e second or third generation stars) of nearby galaxies are consistent with the age of the Universe. Also the observed [alpha/Fe] (thats the ratio of alpha elements to Iron, which acts as a clock for star formation) ratios of GCs and elliptical galaxies are telling us that these systems formed very rapidly in the very early Universe, exactly as you would expect if these "mature" galaxies you are talking about formed very early in the Universe (within the first 2Gyr) and rapidly from many smaller clumps of stars, think along the lines of the spiderweb galaxy and you probably have what is going on.

The fact that galaxies at high z have a high metallicity is neither here nor there, the bursts of star formation that we measure in these systems (objects like the SCUBA submm galaxies) are so huge (of the order 1000 Msun per year) that you very rapidly increase the metallicity without even having to worry about Pop III stars.

From every observation we have the Universe >10Gyr ago was a very different place to what it is today, and the galaxies that inhabit it are mostly very different from today.