If a vendor purposefully under-quoted / undersold me and then asked me to pay more $$ 6 months in, I literally couldn't do that if I wanted to.
The article is quite clear that it is intentionally selling a customer less than what they need in order to get them to buy what they actually need later down the line, once they are already committed.
As a simple example, I could sell you a 3-year software license agreement with a 60 day estimated implementation program on a T&M basis. Then after you have signed the software agreement and are committed to licencing we go into blueprinting, I could suddenly say that it's actualy 120 days implementation now, and that will net the vendor an extra $90k revenue in Y1. Then after implementation you realise that you didn't buy the 'Bi-Connector' that was demo'd to you during the sales process because "oh that's actually another license" even though they clearly showed it as core functionality in their demo.