That is certainly true.
I'm sorry I'm still not quite there - can you take it a little further?
Not sure what you want or need. The plain text reading is they claimed to be Jews but were not. The possible (and plausible) example I gave was of the congregation in Galatia being swayed by someone to believe something that goes completely against scripture: that one must be adherent to all the "law" before they can be saved. It was NEVER this way! Did Rahab even know all the law when she helped the spies, believed what they said and was saved? Yet if I am not mistaken she ends up in the lineage of the Messiah. I know that is disputed, but never really proven false that I am aware of. Regardless, she was saved. And all she knew of the God of these Hebrews was that He was more powerful than all the gods she worshiped. That's all the "law" she needed to convert!
Remember that the letters in Revelation were written to specific people in specific places and at a specific time in history. Those congregations were undergoing trials, and they would have known exactly what is meant, so there was no need for further explanation. We must draw on other scripture to fill in the blanks, so to speak. Unlike others here I label my "fillers" as conjecture, but based in scripture. But I also insist that the plain text reading stands as written- they were saying they were Jews, but they were not. Revelation ends with a warning:
Revelation 22:18-19 ESV 18 I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues described in this book, 19 and if anyone takes away from the words of the book of this prophecy, God will take away his share in the tree of life and in the holy city, which are described in this book.
I take that very seriously.