It's also very possible they stutter but either not in that moment or they've learned to work around their stutter so it's hardly noticeable. I don't usually tell people I stutter if I notice they mention they do too, but my dad will tell someone with a speech impediment that he also has one but you may not be able to tell, or think it's a stutter like fumbling over words, because his stutter comes and goes - perfect fluency some days, really bad stutter other days. I've told people I also stutter and they didn't believe me and dismissed it because it's second nature for me to quickly adjust how I speak to avoid a block I feel coming on. I just think between two possibilities, where one won't make you feel a type of way, it's much easier to defer to what they say happens with them. If it's known for a fact they don't stutter, it'd be different. Essentially what I'm getting at is, in that moment, you have the choice to believe they actually stutter or not. It's much less stress to just believe them, especially when you lose nothing in doing so.