Hi JAE,
Yes. The building is flexible. I have observed that most of the PEB buildings (even though low rise) are flexible building due to lower stiffness of steel framing (at least in portal direction).
Refer to attached where conditions (limitations) for using Envelope procedure are given...
In ASCE7-05, Wind load Section 6.5.12.2.2 is applicable for low rise (<60ft) rigid buildings only (this is clearly mentioned in ASCE7-10 release).
However, all the PEB industry still use Section 6.5.12.2.2 even for flexible buildings (actually almost all PEB building will fall under flexible...
