Your proofreader is not obviously correct.
Like many constructions, this one is a matter of style and not one of necessity.
Although it might sound a little strange, it's not ungrammatical to write it remains only.
I found several instances online where this word order was used and will mention two notable ones:
A seemingly anonymous Italian proverb (here, from The Gold Scales—although it's found at other sites, all with the same spelling and punctuation):
A chi vuol fare, non manca che fare.
To him who is determined it remains only to act.
A legal article titled "Law and Political Reality" by D Zeffertt:
Be that as it may, it remains only to remark that neither Lynch nor Goliath was a case in which the person under duress was the actual killer.
I do find it has a stylistic sense to it that I would not normally use but which actually seems appropriate in the above two contexts (poetic and highly formal).
More interesting, is that Google Books Ngram Viewer shows that it only remains has only a slight edge over it remains only in the written form even though, surprisingly, it remains only, which sounds more archaic, was actually far less common than it only remains in the past.
As a note, if you switch to the American corpus, the two seem to be even closer; if you switch to the British corpus, it only remains has more of a lead in usage.
For an international audience, your preferred usage may be less common. But it's not clearly an error and, strictly speaking, not something a proofreader should be flagging (as opposed to an editor, who is more concerned with style in some cases). Any "advice" beyond that would have to be based on how it fits into the rest of your article stylistically.