If it's cataloged by year of release... still it's strange. Because Caribbean Clipper, Eldorado Fortress was released 1998 / Harbor Sentry, Sabre Island was released 1990. If we follow the timeline we come to 1991 and in this year Broadside's Brig & Lagoon Lock-up came out (not 1992 like you noticed and we may believe Lugnet). All 6 sets are Bluecoats.
In other words for me it would be normal if Broadside's Brig & Lagoon Lock-up also would be cataloged/ fall under Imperial Soldiers (as we follow the timeline). Since the first Redcoats only appeared in 1992 with Bounty Boat, Imperial Flagship and Imperial Trading Post. You understand what I want to say because after 1991 no bluecoat set will be released.
But now Lagoon Lock-up and Broadside's Brig are under Imperial Guards aka Redcoats. If we use the ships as a reference to distinguish which is red (Imperial Guards) and blue (Imperial Soldiers).
Timeline:
1998
Caribbean Clipper, Eldorado Fortress
1990
Harbor Sentry, Sabre Island
1991
Broadside's Brig, Lagoon Lock-up
1992
Bounty Boat, Imperial Flagship, Imperial Trading Post
1993
Cannon Cove
1994
'Imperial Cannon', 'Imperial Guard Camp',
1995
Imperial Outpost