Automotive load dump protection with TVS diodes
Question 1: What makes e.g. the TPSMA6L20A more suitable to this
  application than any standard SMBJ20A? I cannot find any appreciable
  differences in the datasheets.


Immediately the main difference is one is SMA and the other is SMB. This indicates a different package size as well as power handling. However, the TP variant is the high reliability variant from Littlefuse 

The difference here is this part is AEC Q101 qualified

If a component comes with AEC Q101 (or Q100, Q200  for IC and passives) it means certain aspects of the manufacture have a proven quality control plan. Traceability, defect analysis, guaranteed availability for a significant time as well as notice to distributors of any changes 

What you have identified here is the "standard" variant and the "AEC Q101" variant. From a datasheet perspective they should be identical, from a good received the AEC Q101 comes with more guarantees

This is important for the Automotive, aerospace, marine etc industries, but less so for the hobbyist or consumer designers

Whether this part is suitable for your needs... that is a different question 


  Question 2: More generally, is a TPSMA6L20A / SMBJ20A sufficient for
  protecting automotive circuits? Littlefuse speaks of TVS as "secondary
  protection", but they show no primary protection in their example
  circuits. What do they regard as the "primary protection", the Zener
  diodes in the alternator?


The primary means of dealing with bus surges is done at the alternator. This can be classed as the primary protection. This is never enough and there for local per-load protection is needed. This is what is referred to as "secondary protection"


  Currently, most of the alternators have zener diodes to protect
  against load dump surges; however, these are still not sufficient.
  During the powering or switching of inductive loads, the battery is
  disconnected, so that unwanted spikes or transients are generated.


https://www.littelfuse.com/~/media/electronics/application_notes/littelfuse_tvs_diode_automotive_circuit_protection_using_automotive_tvs_diodes_application_note.pdf.pdf