A 1-in-1,000 year event. Only two years after the similar 1-in-1,000 year event that occured two years ago (July 30th 2016). These poor people have only just finished rebuilding Main Street and the Downtown from THAT event.
Alberto certainly assisted by transporting extremely moist air across Maryland and the Mid-Atlantic in general but the extreme rate of rainfall was specifically due to back-building and training of thunderstorms between Baltimore and Ellicott City, so your comparison with Boscastle is pertinent.
Of course, 1-in-1,000 years merely expresses the probability - in other words, a 0.1 per cent chance of occurence in any given year.