A couple things could make this plausible:
1, conditions where these humans live can be such that the current form of homo sapiens is ideal for life there, which reduces the natural pressure to adapt. Why would this be the case? Probably because of an environment at least partly man-made. Clearly in some earlier time, mankind built environments that were ideally comfortable and optimized for them, and their descendants now live in these (probably self-maintaining) habitations which are perfectly suited to their needs. One likely reason for this to go on so long is space colonization, which would force humans to live in completely synthetic environments or "terraformed" environments or giant dome areas which were specifically tailored and engineered for their comfort.
2, There could be a "baseline" genetic reference which keeps being reintroduced into the gene pool. This could also be the product of an earlier, more advanced phase of humanity. Shortly after perfecting genetic manipulation, humans may have preserved specific genetic "libraries" of human genes with what were considered desirable (probably profitable aka: "designer") traits. Sophisticated, automated systems can replicate this genetic stock which is still available for use, and probably includes some cool new or optional "features" which people might still find desirable, but to use the old stock, they must be close enough to "baseline".
2.5, Alternately, these "baseline" genes may be introduced from clones which are based on old-style genetic stock. There may have been a legal loophole which allowed clone "slavery" or servitude if the clone was created from scratch and grown entirely in a lab. Perhaps a "second class citizen" type system more like the highly educated Roman slaves who ran the empire's administrative apparatus. Over time, for legal/humanitarian/political reasons, the stock from which these clones could be produced was frozen to one basic "pool" and production continued for some considerable time. Inevitably, the descendants of the clones would mix with the non-clones, pulling the entire human gene pool back toward "baseline". Also, if there were cultural effects where non-clones were considered "superior" for some period of time, and there was a long term, low birthrate, the higher class strata may have faced a milder version of the inbreeding problem faced by European nobility during the Renaissance. This would make it necessary to integrate clones into the gene pool as well. After that kind of a long-duration imperial structure collapsed, humanity might be very close to baseline. Even farther out, perhaps the cloning facilities were kept active for religious reasons in some post-imperial religion where creating a clone based on "baseline" (ancient) genes was thought to bring back in some spiritual way the virtues of ancient humans from before the time of imperial decadence.
Basically, most of this would all boil down to some very sophisticated self-replicating and repairing machines which had been developed by humans at the height of their development, which continue to function.
Edit: since I answered, yet another plausible scenario came to mind which might even make a more interesting story. What if humanity (that is, 20th century humanity as we know it) has actually gone completely extinct and then been reintroduced? This could be either through a quasi-conservation program similar to current efforts to clone a woolly mammoth, or through some high tech "worst case scenario" fail-safe mechanism that was reactivated and began cloning modern humans again. Even if this event occurred far in the past in your future timeline, it would still put modern humanity far into the future.