Not sure why the concern with the tree; I do not share the concern tbh. If I look at the pictures of the branches trimmed last year they seem to have good size & foliage?
I've always been told when you collect, to allow the energy of the growth to be left untouched. Especially if its not robust growth. This species is a weed in my area. My uncle propagated them for years putting them all over farm. I've never seen such lack of growth in a years time. I've always heard ones say not to prune trees that are not showing signs of strength. This little bit of branching...for the species. Knowing how robust they are in pushing growth. I just wouldn't have pruned it. But...I grew up watching this particular species more than any other shrub. Because of my uncles fascination with it. He absolutely loved Forsythia. This trees output of growth...was minor, not typical of a growing season.
I mean it's done been pruned back...will it kill it. Most likely not. But I would not prune it again until it was more robust with energy and growth. To establish a better root system under it.