There's no way for us to know just from your description what was wrong with the derailleurs or chain except possibly if you showed a photo of them and gave more accurate details about the actual problem.
If you do a googgily search, you'll find some people asking the same question about a stuck RD fully extended on the largest cog. Some guesses were too short chain, B-screw not screwed in deep enough, chain suck, etc.
https://bicycles.stackexchange.com/q...rgest-rear-cog
https://forums.bikeride.com/thread-5191.html
https://forum.bikeradar.com/discussi...ech-locking-up
Whether or not the chain length was the problem, you can tell if it was too short by looking at it and seeing if the RD cage was fully extended when on the largest cog. It should not be.
The FD doesn't have to match the RD brand. All the FD does is push the chain on or off the chainrings. What's more important is that the shifters match the correct indexing/pull ratio of the FD or RD.
If you didn't do any maintenance for 5 years, you would need to do some. Periodic adjustment of the alignment of FD, RD whenever bumped, crashed, or experiencing shift problems. Cleaning of chain ( and maybe disc brakes less often) maybe every hundred km, 2-4 weeks, or when riding in wet weather.
If you rode a few thousand km without cleaning the chain, it could be stretched out too far. If you have the same cassette and chainrings for 5 years, depending how often you were riding, they may be worn out. You have to check them for wear.
If you asked him to only change the brakes, I don't see why he would have to inspect your other components.