After some experiments: everything works OK when opening .sla file on computer without any .icc profiles installed on it. So Scribus does some color converting (tried on two computers, with Windows and Linux and Scribus 3.3.3.12 on them) even when color management is turned off.
When Scribus finds no icc profiles it doesn't change any image's colors and output file are just like I wanted them to be ;) (similar effect with printing to postscript file and then ps2pdf it - with Scribus with icc profiles).
