Not sure if I am reading the rules correctly,

3.3.1 Limits
Each franchise will be allocated a Developmental Taxi Squad (DTS) containing up to 5 players. Players on a team's DTS in a prior year, along with any rookie additions, may be placed on the DTS. A player may remain on the DTS for a maximum of 3 years. The deadline for announcing DTS players will be in accordance with rule 3.2.3 final roster declaration. Any player on an active roster after the final roster declaration is no longer eligible to be placed on any team's DTS. For the first season, any player with less than three years NFL experience may be placed on the DTS. This exemption does not allow players to stay on the DTS past their third year of NFL duty.

3.3.3 Miscellaneous
DTS players do not participate in RFA. FA’s may be placed directly onto the DTS providing they are in their rookie year and have never been on any LLD Active Roster. Traded players may be put onto the DTS only if they were on the other teams DTS before the trade. The DTS concludes at the end of the playoffs.

There seems to be a couple players on Taxi squads that were on LLD rosters, for example Dominique Edison and Brian Robiskie
were both on rosters and giving contracts, however they are now on the Taxi Squad.

One other thing I noticed is Tyler Thigpen on Randells roster have no contract but is not on DTS.

he's not eligible for DTS, so yeah, a contract is needed.

Emailed Randall should get a response shortly