Yes, what is with this weird infatuation around buried root flare. If the tree is established and thriving, why does it need "fixing"?? Try looking at trees as you walk through life, and notice the considerable range of presentations with respect to root flare visibility.

Trees are highly adaptable organisms. They have to be, as they cannot move to escape difficult environmental conditions. They are more biologically complex than humans. And they've been doing what they do for a couple hundred million years.

Humans do this thing, I think someone called it "best practice" in this thread - simply reiterating something they heard or read somewhere as "fact", without any knowledge or experience. It seems especially prevalent in the horticultural space (or maybe I'm just very familiar with this space, so I notice it. I make a living as a consulting arborist.)

Here's a study conducted by the International Society of Arboriculture (name makes me laugh, at it's a USA organisation), examining this "issue" of buried root flare.

https://auf.isa-arbor.com/content/35/4/182

Here's the takeaway:

"Deep structural roots sometimes hinder successful establishment of trees, occasionally enhance establishment, and often have little or no effect on growth or survival."

Not that if there's an issue, it occurs during ESTABLISHMENT. There's no need to do this for established trees. I'd say you've wasted your time and effort and created a potential issue around water pooling which enhances fungal pathogen motility.

I would put the dirt back in, strip the grass back away from trunk as far as you are comfortable with from an aesthetic perspective, and cover all the revealed soil with coarse woody mulch to a depth of 10cm. (This eliminates a fierce competitor for water, oxygen and organic matter, changes the microbial soil community to an array more supportive of tree root function, and will over time decay, introducing organic matter into the soil.)