...He has no signs of founder at all and has been running around the pasture happily like he always does.. The relationship between founder and laminitis confuses me though...
Unfortunately the pics aren't helpful. You need to at least take close up, non blurry pics for critiques, and if you check out the link in my signature below it is to a website with helpful info on what's needed for critique shots. What you describe sounds like it could be just swollen periople(coronary skin) from too much wet footing. Is it soft, the swollen bit, or is it actual wall material that's bulging?
Founder & laminitis are often used interchangeably, 'laminitis' being the more technical term & 'founder' the lay term. There is 'low grade' or 'sub clinical' laminitis/founder, without obvious lameness. There is acute laminitis, where the horse has a bounding pulse & painful feet, may want to lie down all the time or rock back onto heels when standing. There is mechanical laminitis/founder with 'rotation' or 'sinking' of P3 within the capsule, and many other degrees & specifics. Often though, people use the terms differently, and 'laminitis' refers to the inflammation/pain, while 'founder' refers to the generally chronic(long term) mechanical changes that happen to a laminitic foot that is not managed adequately.
IMO it is so vital to learn about stuff like laminitis, as a horse owner, even if you own a 'laminitis proof'
TB or arab! The reason being is that EVERY horse is at risk of laminitis, due to many reasons, not just obesity & OD of grain, and that people(even vets & farriers) still frequently don't recognise signs or worrying factors until 'suddenly, out of the blue, for no reason' the horse is acutely lame, or wearing 'ski feet'. While if they understood about 'low grade' laminitic signs, they could avoid the vast majority of serious & chronic cases.