The altitude at the marker is a double check on the
altimeter and the glide slope. If the altimeter is set
incorrectly or broken or if you have intercepted the wrong
glide slope lobe, the pilot has an opportunity to catch the
error and figure out what is wrong.

That is of course true - but it's a different altitude, 1758 in this
case. It's the 1800 altiitude that loses significance once the GS has
been intercepted.

BTW, the chances of following the wrong lobe of the GS to the FAF
without noticing a problem is close to nil. It either has reverse
sensing or requires ridiculous rates of descent.