Reformed Baptists believe there are Reformed Baptists. Reformed people say there is no such thing.
I'm not trained historian, but this is my take on it from what I've read. Feel free to tweak my history if this is your area of study. Historically, early on in the Reformation that is, the Reformed group believed in salvation by faith but did not agree with Luther in consubstantiation-- Luther's belief that while the bread and wine became the body and blood like the RCC taught, that it was also still bread and wine. That was the historical difference between Lutheranism and Reformed early on, back when Zwingli was the key leader.
John Calvin then became the central Reformed leader and theologian. He emphasized the sovereignty of God. Some time after Calvin, the theology got packaged into the 'TULIP' acronymn to summarize Calvinist theology. TULIP doesn't address the issues of transubstantiation, consubstantiation or a spiritual presence in the eucharist, so are TULIP Lutheran's Reformed?
The Reformed groups also believe in infant baptism. Baptists don't, so the historical Reformed people may not accept Baptists as 'Reformed.' The first time I read 'Reformed Baptist', I was thinking 'huh?' and scratching my head. I'd never heard of that. Then I read something from a Reformed person online who wrote that the title 'Reformed Baptist' was rather recent. Is it just a couple of decades old? It refers to Baptists who believe in 'Reformed theology' in terms of soteriology, excluding doctrines related to water baptism, right? It seems to be a new title developed during this recent new Calvinist movement among Baptists and similar groups.