They note that the process is so quick because there was already a large untouched forest next to it. So any deforestation next to a forest can be fixed quickly, but it would have to creep its way inward over a longer time if there was no forest nearby.

In some cases, yes. Ecosystems are resilient to moderate damage but almost impossible to fix after severe damage.

Normally you will not have a winning combination of very fertile soil, water, forest near to provide recruitment of plants and animals and private land with strictly not allowed arsonists of any kind.

Fifty years is a heartbeat for an ecosystem.

England was deforested. And so was New England. People were burning wood and making charcoal.

Then coal and oil took over, and the forests regrew.

Edit: my experience in Northeastern Massachusetts is that it takes about fifty years for cultivated land to grow up into fifty foot white pines. In another hundred years the climax forest of oak, ash, maple, and beech would take over.

It will depend on if the farmland still has viable soil and water. If the topsoil has blown away or if it is now desert relying on irrigation, it is going to take longer to regenerate. You also end up with things like salt, where water mismanagement has caused a high level of soil salinity and you are going to need specialist plants to regenerate.

40 years of ecosystem recovery from a vantage point (Landsat) in a 6 minute video about Mt St Helens post 1980 eruption: https://svs.gsfc.nasa.gov/13614