I think for him, and also me when I'm doing strength training, it's a matter of just keeping the overall system balanced and stable versus getting swole. I would absolutely isolate more muscle groups on more days for swole purposes, but when I'm marathon training I have a lower body day and an upper body day just to keep everything happy.That's like an upper body/lower body split then? Most people I talk to do core work on leg day. Is there a benifit to doing core on the same day as uper body?
ETA i do core work post-run during marathon training. I do zero extra conditioning work during tri season.