Actually, for most of Constantine’s reign (the sole western emperor after his defeat of Maxentius in 312 until 324) he was the Western Emperor, while Licinius was the Eastern Emperor.

The biggest problem with this claim is that Rome at that point was already not the administrative capital of the empire. The emperors of the later third century, especially after Diocletian, were much more itinerant and when they did stop for a winter and have their capital, it was at cities like Mediolanum (Milan), Augusta Treverorum (Trier in modern-day Germany), Nicomedia (modern-day Izmit in Turkey, Sirmium (in Illyricum, modern-day Serbia), and other border areas where the reigning emperors could more easily respond to external and internal military threats. Rome really wasn’t functioning as a capital city at this point. This is not to say that it was declining rapidly, far from it. It’s just that the administrative primacy of the city was already largely gone and so we have less evidence for lots of money consistently being spent on the city by emperors (this is not to say that important expenditures like the grain dole did not continue to happen; it seems like OP’s guide was talking primarily about building, which does continue, but not as intensely as in earlier periods: see below). Yet, it was still probably the largest city in the Empire before and after Constantine’s reign.

One of the periods where there was a lot of investment in new buildings was during Constantine’s reign. In the early part of his reign the Emperor/Pretender Maxentius spent lots of money building a palace complex along the Via Appia (which is still there and I would strongly encourage people to go visit), as well as a massive basilica in the Forum. Then after Constantine’s defeat of Maxentius, he complete’s Maxentius’ forum basilica (and slaps his name on it), and spends a lot of money on building in the city, most notably by building massive churches (Old St. Peter’s, St. John Lateran, St. Sebastian, and St. Lawrence Outside-the-Walls), and to a certain extent re-orienting the cityscape more towards Christianity (though I hesitate to say this because it has been over-exaggerated by historians in the past). Essentially, not only did Constantine not “abandon” Rome, but he spent way more on building there than did his predecessors.

As to the taking of artisans and architects to Constantinople, I know of no source that assumes that all of Constantine ‘s new architects were taken from Rome. Indeed, it would be reasonable to assume, given the large investments in building at the other imperial capitals, that the Emperors had architects attached to their mobile bureaucracies. If you want to look at places where patrimony was taken, Greece is a better example, as Constantine has antiquities (primarily statuary, but another example is the trophy at at Delphi for the Persian War victory in the 5th c. B.C.) taken from there and transported to adorn his new city. Constantine did impose incentives to move to the new city, and there are members of the aristocracy who move to the new Constantinopolitan senate, but Rome’s senate still remains influential well into the 6th c., even after the “Fall,” so that’s not the reason either (also we have references in law codes to the need to increase the size of the Constantinopolitan senate and references to allowing wealthy but low born people into the senate to the disgust of some senators, so he actually had problems enticing a lot of people to move).

Basically this statement by your guide was all kinds of wrong:

TLDR; Rome was already not the Capital by the time of Constantine; Rome continued to be an important and influential city after him; Constantine spent a lot more money on the city than his recent predecessors; No evidence for large-scale migration of people from Rome to Constantinople