Walk in walk out works the same as any other approach I spend 2-3 hours walking in and out every 10-15 minutes or so when she starts screaming hysterically tuck her back in tell her goodnight and walk out and within a few minutes whimpering starts back in and escalates over the next 10-15 minutes to full blow screaming till I go back in and do the same thing.
OK to me the way you have described it really sounds more like controlled crying than it does WI/WO, for me the WI/WO would happen when they whimpering gets to be more than a whinge. To me it sounds like there is an issue with separation anxiety going on. I will admit I totally struggle with hearing that a baby or toddler has been left until they get to full blown screaming as I'd never leave an 11yo screaming, so why would a 2yo be left screaming. I do understand that as parents we need sleep, but not at the expense of a child getting hysterical or screaming & I think that when they get to that point then maybe it is overlooking the emotional needs of a toddler in the needs of an adult to have them sleeping.
To me WI/WO is more about going in the moment a child shows that they need comfort (big whimper) & giving it to comfort them to show that Mum &/or Dad do come to their call & that we would never leave them. If they are left until they are really upset, well I struggle with that because I know with older children how hard it is for them when I was even a few minutes late picking them up or situations like that & think how stressful it must be for a 2yo to not really trust a parent was coming back.
If she settles when you stay in the room, personally I'd set myself up for that for a week or two & re-build trust & see if that doesn't improve sleep.