The lack of any other rewards is annoying, but acceptable, as you say there was never any mention of them. The Ranking achievements (It's My Party, Party Of Two (to Ten), Party Of Eleven (to Thirty)) are also somewhat understandable, as the rankings for Team PvP in Season 7 were never published, although I'd be surprised if SE don't have the data for them.

The Tier achievements (Diners' Club: Bronze, Diners' Club: Silver, Diners' Club: Gold, Diners' Club: Platinum, Diners' Club: Diamond) however had entirely fulfillable requirements. I don't know if anyone reached platinum or diamond this season, going by previous team seasons results Diamond is unlikely, but regardless if they finished the season in any team tier the corresponding achievement should have been awarded.