In regards to the similarity scores, him and Delgado both have 3 players on their list already in the Hall of Fame, (In Thomas' case, those three are Mickey Mantle, Willie McCovey and Jimmie Foxx). Like Delgado, many of the similar players not enshrined are either active or haven't been retired long enough (Bagwell, Ramirez, Thome, Sheffield, Delgado, etc.)
Like Delgado, the fact that home runs were inflated in the 90s hurts in that it lowers the impact of home run totals regardless of how clean or not clean the given power hitter was. In terms of total home runs, Thomas is tied for last but isn't that far behind (all 4 players hit between 521 and 536 career home runs).
In context of OPS+, Thomas is third out of 4:
Mantle 172 OPS+
Foxx 163 OPS+
Thomas 156 OPS+
McCovey 147 OPS+
One thing that supports Thomas' argument is that for two consecutive years (93-94) Thomas won himself back to back MVPs. Thomas is square in the middle of discussion here as the other similar players already in won between 1 and 3 MVPs.
When you extend it out to Top 10 MVP finishes, it becomes even clearer that Thomas was a very valuable player for a very long time. He finished in the top 10 for MVP voting 9 times. That ties him with Mantle and allows him to outpace McCovey and Foxx.
If you read enough of my writing, it's quickly obvious that OPS+ is one of my favorite stats to use to measure how good a player actually is. Thomas passes the OPS+ with FLYING colors. In his first 8 seasons his lowest OPS+ was 174. In his EIGHTH best season he still hit 74 percent better than a league average player. He was so dominant during this stretch, he even had a Super NES game named after him in the 1990s. If that don't spell dominance I don't know what else does.
In their entire careers, Mantle, McCovey and Foxx had between 4 and 8 seasons with an OPS+ of 174 or greater. None of these players had all of these seasons in one uber-long dominant stretch. It seems a bit unfair that this run of Frank Thomas was obscured by the inflated power numbers of the 1990s but for those first 8 years he was one of the most dominant players in the game, he was THE Big Hurt. To put icing on the cake, out of the 4 Hall of Fame metrics measured by Baseball-Reference, Thomas is measured as a likely Hall of Famer in 3 of them.
Unlike Delgado, there isn't much of a gray area for Thomas, his body of work (especially that 8-year stretch in the beginning) screams first ballot Hall of Famer to me.