Technical answer:

29 CFR 1910.134 (b) has different definitions for “air-purifying respirator” and “atmosphere-supplying respirator.”  In other paragraphs, such as (c) and (d), the term “respirator” is used.  In (d)(2)(i), the terms “SCBA” and “SAR with auxiliary self-contained air supply” are used.  Paragraph (h)(3)(iii), in specifying inspection procedures for SCBA, the pertinent sentence seems to me to be:  “Employers shall determine that the regulator and warning devices function properly.”   I argue that flow testing or other manufacturers’ requirements must be met to determine that the “regulator  . . . function(s) properly.”

Non-technical  answer:

It’s 8:30 pm on a Friday night and there is either a visual confirmation that there is a release of a hazardous material (puddle or pile of stuff on the floor at or near a broken container, hose from a compressed gas cylinder is broken and venting, etc.) or an alarm is sounding.   If you are the one going to clean up the mess or otherwise respond, how sure do you want to be that the appropriate respirator is working properly while you do initial monitoring and get containment or confinement of the spill/release?  Thud (as in fall down and go thud) is not a reliable of “function properly.”