I think they mean that it is likely to heavily depend upon the task.
Decoder models are good at generation of language, and of course they're going to do well where that counts.
But if you want to do typical NER+Relation extraction and then normalize to an in-house dictionary of 10 million IDs? You can't do that as effectively with GPT-4.
You need a local model (right now).
There are a lot of things that GPT-4 doesn't touch in terms of data domain, so for many projects, yes local (typically encoder) models are 1) way faster 2) way" cheaper and 3) have better metrics.
Of course, the capability
could* be there with a large decoder model, but if it's a task that needs your specific (large amount of) data, you have to make something locally.