What word is "inexplicable" posing as in this sentence?

Background: I post occasionally on Next Door, in a neighborhood which is well educated. I ignore typos, and even mistakes such as your for you're and it's for its. and, with less good cheer, imply for infer and vice versa. These are busy people (no sarcasm) and their fingers can outrun their brains, and I make mistakes too. At least it is clear what they mean.

But one recent mistake has me puzzled. A distraught OP told us how a driver hit her dog, and did not stop. The dog may or may not survive after a long surgery. One comment was:

I am inexplicably sad to hear this.

The rest of the post made it clear that there was nothing inexplicable about her sadness. What word would have made better sense? The only thing I can come up with is inexpressably sad. Is there a word that might be easier to confuse with inexplicably?

She probably meant "ineffably", which MW defines as:

• ineffable" (adj)
  a. "incapable of being expressed in words", indescribable, eg ineffable joy.
  b. "unspeakable", eg ineffable disgust

inexplicably

In a way that cannot be explained or accounted for. Lexico

In an inexplicable manner; for an unknown reason. Wiktionary

It's possible that the OP used this word because he or she can't explain their depth of feeling when thinking or writing about the event. I don't think it was a mistake, even though we would normally use the word when we don't have a real reason to feel sad. In other words, something like inexplicably much sadder than I would ever imagine.