I tend to disagree. And one reason why, take John the Baptist and Justin Martyr, for instance. Both were literally beheaded, but neither were beheaded because they refused to worship the beast, neither his image,etc. There was not yet a beast to worship or not worship at the time. There was not yet it's image to worship or not worship at the time. But there was such a thing as being martyred at the time for the witness of Jesus, and for the word of God.
What I tend to think, the martyrs resting under the altar during the 5th seal are meaning these in Revelation 20:4---that were beheaded for the witness of Jesus, and for the word of God---and that their fellowservants who also are to be killed, are meaning these in Revelation 20:4---which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands.
What I also tend to think, being beheaded can be applied both literally and non literally at the same time. In the case of someone such as John the Baptist and Justin Martyr, it is meaning literally. In the case of someone like Stephen, it is meaning non literally. Think about it, what does it mean to be beheaded? It means to be silenced, it means to no longer have ones voice heard on earth. And that's exactly what happened when they stoned Stephen, that from their perspective, they silenced him, they no longer had to listen to him.
One thing that some Amils need to keep in mind as well, since some Amils I have encountered in the past have claimed that when Stephen was stoned, it was because and during the time of the beast in Revelation 13, yet we are told the following which refutes that---Revelation 17:8 The beast that thou sawest was, and is not; and shall ascend out of the bottomless pit, and go into perdition: and they that dwell on the earth shall wonder, whose names were not written in the book of life from the foundation of the world, when they behold the beast that was, and is not, and yet is.
Which period of time did Stephen live in? Obviously this period of time---and is not. No matter how one looks at it, until we enter this period of time---shall ascend out of the bottomless pit--there cannot yet be any martyrs who refuse to worship the beast, neither his image, neither have received his mark upon their foreheads, or in their hands. Until we enter that period of time we are still in this period of time---and is not. The question that begs an answer, which period of time are we currently living in, then?