While sifting through the source material at hand, I realized that before documenting that match I needed to tackle the Sixth American Chess Congress. That brought me to the door of one of the mysteries of the early World Championships. Here's a writeup from Wikipedia:-
Under rules that reigning World Champion Wilhelm Steinitz helped to develop, the winner was to be regarded as World Champion for the time being, but must be prepared to face a challenge from the second- or third-placed competitor within a month. Mikhail Chigorin and Max Weiss tied for first, and remained tied after drawing all four games of a playoff. Weiss was not interested in playing a championship match, but Isidor Gunsberg, the third place finisher, exercised his right and challenged Chigorin to a World Championship match. In 1890, he drew a first-to-10-wins match against Chigorin (9-9 with five draws). These were the same terms (9-9 draw clause) as the first World Championship match between Steinitz and Zukertort in 1886. Incidentally, they were also the same match terms that Bobby Fischer would insist on for his title defense in 1975. Sixth American Chess Congress (1889)
Note the phrase 'the winner was to be regarded as World Champion for the time being'. This is contrary to what I knew about the Congress, and I assumed that the anonymous Wikipedia writer simply got it wrong. After all, I've seen similar errors before. Another well known web source on the World Championship explained things differently:-
This tournament was, in a sense, the first Candidates tournament. In addition to naming a US Champion, the organizers planned to finance the winner in a World Championship match against Steinitz. Max Weiss and Mikhail Tchigorn tied for first, and remained tied after drawing all 4 games of a playoff. Tchigorin was not interested in challenging Steinitz again so soon, and Weiss was not interested in playing one at all, and so the plans came to nothing. However, Isidor Gunsberg, the 3rd place finisher, was interested. In 1890, he drew a 10 wins match against Tchigorin (9-9 with 5 draws). Because of these two results, his challenge to Steinitz was accepted, with their match being played in 1890-1. Sixth American Chess Congress, 1889 (graeme.50webs.com)
The phrase 'in a sense, the first Candidates tournament' was closer to my understanding, but the issue of who would challenge whom provoked another question mark.
This brought me back to a footnote in the Wikipedia article, a link to Steinitz — Chigorin, Havana 1889 - A World Championship Match or Not? by Anders Thulin, a well-documented essay that drags the Steinitz - Chigorin match into the discussion. Wikipedia dates the Thulin essay to 2007, while the current version is dated 2009. This means that the document requires careful re-reading and a comparison with other sources from that era. I'm not prepared to do that now, but will find the time as soon as I can.