yes the sill can be welded. take the car to a car body shop & get a price as car body shop can usually do a better weld finish than a garage.
if it's passed the mot then they must feel the rust is superficial to pass it. some garages put down anything on advisarys but i would get it looked at as it may fail next yrs mot & rust quite a lot in a year once rust has set in.
my boys fiesta had not advisarys last year and this yer it faild on the sills rusting & had to have a complete new sill one side & plated the other side.had it done in a car body shop & it's looks brilliant. dont look like they done any welding to it as they use a proper undersill to coat all the weld and all the sills so it looks original not a patched up job.