An isolationist -- if words have any meaning at all -- is someone who wants to isolate the country from interaction with the rest of the world. This isn't simply about disengagement from foreign conflicts but also drastically reducing our trade relations around the world -- and perhaps, initiating trade wars. So the real isolationist is someone who wants the military budget limited to what's required to defend the country, but who also wants national self-sufficiency and isn't interested in diplomatic engagement. Put this way, very few people would qualify as isolationists. Put this way, can you think of anybody who actually takes a totally isolationist view?
They were in favor of non-interventionism, which is a different thing.
Non-interventionism means that while one does want to isolate the country from foreign conflicts, one doesn't want to roll up into a ball and sit in the corner. Non-interventionists don't want to intervene in the internal affairs of other countries and don't want to get involved in wars that are none of our business. Non-interventionists want to define what is our business in a reasonable, non-insane kind of way. After all, not everything in the world is our business.
In other words, the non-interventionist wants to follow the advice of John Quincy Adams:
Wherever the standard of freedom and independence has been or shall be unfurled, there will [America’s] heart, her benedictions and her prayers be. But she goes not abroad in search of monsters to destroy. She is the well-wisher to the freedom and independence of all. She is the champion and vindicator only of her own.