Glock has been making components and complete pistols in the US for some time now and Austrian production is not being phased out.
Tenifer is simply a trade name for a chemical process that is done in the US under the trade name Melonite. Only a materials engineer can intelligently explain the difference so that shouldn't be an issue from a consumer's standpoint. Either process is just a surface treatment to the metal - an applied finish (like Cerakote) is what gives the slide its finished appearance.
The quality of the US made pistols is indiscernible in appearance or performance.
I carefully looked over an Austrian and a USA made Glock and there is a difference, IMO.