Although a 757 is in the medium category (cut-off is 300,000lbs / 136T MTOW), the advice is that it's treated as heavy due to it's particalurly powerful wake vortex. The same applies to the B707, VC10 and IL62. I can personally vouch the VC10 has a very powerful wake vortex, and is in the heavy category anyway.
I'm not sure where this is written officially, but it appears in the AERAD supplement.
If the RJ did use an intersection, three minutes delay should have ben applied.