This is the same bone flip import issue ive had with GoldSrc models for a while now and have previously brought up in bug reports twice now:
https://psteamcommunity.yuanyoumao.com/groups/BlenderSourceTools/discussions/0/2626094856506910191/
https://psteamcommunity.yuanyoumao.com/groups/BlenderSourceTools/discussions/0/1744479698794920993/
I hope this gets visibility since this has been ongoing

Ill also repeat for anyone reading my hacky fix for now:

1. import ref mesh, this will look fine but of course is bone flipped
2. in notepad open the ref mesh smd, and copy everything from the start all the way till the end of the skeleton block end (do NOT go into "triangles".)
Paste that into a new document and save as "posefix.smd"
3. back in blender import "posefix.smd". It will now look broken, but go into pose mode, select all then "apply pose as rest pose".

This issue exists in the old 2.79 and current BSTs and affects many models.

Can also confirm this happening in Source engine models too. Gmod Citizen C_arms, the right forearm flips without knowing on import. Spent a huge amount of time troubleshooting to find its once again this same issue.