> Are they just exaggerated PR verbs, or am I just too old for this?
False dilemma.
They are proper nouns.
Beacon chain is simply the name of the network launched a bit over a year ago. It is likely intended to be named as an aspirational thing to move toward.
Mainnet is the network launched 7 years ago. Usually distinct from "test networks", testnet, and seen far beyond ethereum into all cryptocurrency networks, and inherited from the idea of test, development, staging and production environments seen in the best practices of all web projects in the broader tech sector.
This is an article about them merging.
What is "weird pretentious verbiage" about that? If there was anything odd it would be "the docking" and that article immediately says that was a former name, making it a moot point.