Yes, we should interpret the text literally.
But we should also translate it correctly.
The verb form εἶπον, eipon (a form of the common verb λέγω, lego) seems to require special treatment, according to Thayer.
εἶπον, 2 aorist active from an obsolete present ἘΠΩ
a. equivalent to to asseverate, affirm, aver, maintains: followed by an accusative with an infinitive,
My understanding of what Thayer gives (on the whole of that page referenced in Biblehub) is that this particular form of the common verb should be seen as an 'affirmation' or as an 'averance'.
It is more than just an utterance, or just an idle comment.
It has force, it has intent, it means something.
It is in this sense that the apostle is using the word, meaning that nobody who, as a statement, as a testimony, as an affirmation, says "Lord Jesus" (as is the original) can do so without the inspiration of the Holy Spirit.
ουδεις δυναται ειπειν κυριον ιησουν ... ει μη εν πνευματι αγιω [TR]
No-one . is able . to say (infinitive) . Lord Jesus (accusative).... if not in Holy Spirit.
The construction is a present tense, an infinitive and an accusative singular.
The interpolation 'is' (Jesus is Lord) is not present in the original. The apellation 'Lord Jesus' is, therefore, as an accusative, to be seen as a vocative.
The person being quoted is addressing the Lord Jesus, personally.
Nobody addressing the Lord as 'Lord Jesus' in a way of affirmation, in a way of 'avering' (as Thayer puts it) can do so save 'in Holy Spirit'.
No-one is able to aver 'Lord Jesus' if not in the Holy Spirit.