The Young modulus of PTFE is about 500 MPa and the surface of a PTFE tube for 1.75 mm filament is about 9.4 mm2.
The Young modulus of PLA is variable, between 350 and 2800 MPa, let's say 2000 MPa. The filament surface is 2.4 mm2.
The ratio is about 1 because the surface ratio is 4x and the Young's modulus ratio 1/4x. As result, the stretching of PTFE is about similar to the compression of PLA inside it (of course they sum, since they go in opposite directions). PLA is very stiff: other materials will compress much more and make the stretching of PTFE less relevant.
However the filament is thinner than the tube, so it will curl a bit, which increases its contraction much more! It is likely that even for PLA the stretching of the PTFE is much less relevant than the compression of the filament.
Anyway, fiberglass has a Young's modulus of about 80 GPa (150 times PTFE), but its thickness is what? 0.1 mm? That bring a surface of 2 mm2 at most (and I'm very optimistic, more likely much less than 1 mm2). Fiberglass helps to strengthen PTFE by a factor of surely less than 30, more likely 10 or less (will the glue hold or slide?).
As result, PTFE, from a contribution to the overall stretching/compression of 50 % (in fact, much less due to curling), will be reduced to about 5 %. It is good, but this is an optimistic value.