Here's where I would pinpoint the main mistake in the
paper.
From the paper we have:
The first relation ##(x_{1-\mu}' = 0, t’ = t_1') \equiv (x_{1-\mu} = L, t = t_1)## gives the primed and unprimed coordinates for the event of the muon creation at the top of the atmosphere. This looks correct if ##t’_1## and ##t_1## are defined to be the time of this event in the muon frame and earth frame, respectively.
The second relation ##(x_{1-E}' = -L’, t’ = t_1') \equiv (x_{1-E} = L, t = t_1)## is obscure and appears to refer to some event different from the event of the muon creation and different from the event of the muon arriving at the earth’s surface. If we accept the left side of this relation, this event takes place on the earth's surface at a time ##t_1’##
according to the muon frame that is
simultaneous with the muon creation event at the top of the atmosphere. Apparently, the author doesn't realize that this event is distinct from the event of the creation of the muons. He takes both relations given in the quote above as "for the event of the muons creation".
We could imagine a firecracker exploding at the earth’s surface at a time in the muon frame that is simultaneous with the muon creation. Then ##(x_{1-E}' = -L’, t’ =t_1')## would be the correct primed-coordinates for this event. But, then, we see that
the right side of the relation cannot be correct. The right side says that the unprimed coordinates of the firecracker event are ##(x_{1-E} = 0, t = t_1)##. The author has
assumed that the unprimed time coordinate for the firecracker event is the same as the unprimed time coordinate of the muon creation (##t_1##). That is, the author has assumed that the muon creation event and the firecracker event are simultaneous events in
both frames. However, this assumption contradicts the relativity of simultaneity incorporated in the LT equations between the primed and unprimed frames.
[In the paper the primed (muon) frame moves in the negative ##x##-direction relative to the unprimed (earth) frame. So, if ##v## denotes the relative speed of the two frames (a positive number), the signs in front of ##v## in the paper’s LT equations are wrong.]