A true oversold J cabin is a rarity and will cost UA thousands to "fix" at the gate (presuming it was the over the ocean leg) so this was for sure a low-ball approach. I have had this happen once in the last ~10 years on a LAX > LIH leg of an ORD > LAX > LIH trip. Even as a GS at the time an agent proactively called to tried to get me to swap to ORD > HNL > LIH. My wife hates the island hoppers so I declined but they offered me $3,000 ETC ($1,500 each) to swap because they were over by 2 on a 752. I asked how that even happened and the GS representative basically said they had irrops with some mega-VIP and that this was now dropped in their lap.
This is not entirely a rarity, as I have seen it on a few occasions in my own travels and reports in from family members. I have seen SFO-SIN oversold at the gate by 1 in J, and UA lucked out with a missed connection to even the score. Have heard rumors that UA is willing to oversell J by 1 on some routes... doing it for SFO flights doesn't surprise me given how many missed connections I see there. It's a big win for revenue mgmt to get away with it consistently and not lose a J seat to an upgrade instead of a full fare ticket. This being said, I'm not saying it's common, but it's not super rare either.