The issue was the missing  ` -x ` .. which is the correct "translation" of "export ISE_EIFFEL=.."  in fish shell.

i.e

set -x ISE_EIFFEL /home/eiffel/opt/Eiffel_19.05
set -x ISE_PLATFORM linux-x86-64
set -g -x PATH $PATH $ISE_EIFFEL/studio/spec/$ISE_PLATFORM/bin

In additon, I would suggest to add the following paths to the PATH
   $ISE_EIFFEL/tools/spec/$ISE_PLATFORM/bin
   $ISE_EIFFEL/library/gobo/spec/$ISE_PLATFORM/bin

Regards