This does not have anything to do with the level. Ive had this happen with level 2 and 3 purges as well. The purge bases spawn just a few feet from my base cause me to aggro not only the archers but also the 3 spearmen inside the camp.
Ive lost 4 T4 thralls to this lol. Its pretty sucky when it happens but I have not been told if this is a bug or working correctly.