Would Airport Capsule be holding data from external SSD?
Yes, if you set up Time Machine to back up both your Mac and the attached SSD.
Chances are, you would remember if you did this.........you would have set things up as follows:
Open System Preferences (the gear icon on the dock)
Open Time Machine
Click on Options
By default, the SSD would appear here as an item that was Excluded from Time Machine backups
IF.....you selected the SSD to highlight it and then clicked the - (minus) button at the bottom of the Exclusions list to remove the SSD from the list.....then....the SSD would be included in backups and Time Machine would have backed up both your Mac....and...the SSD.
Do you remember setting things up this way? If yes, you should be able to restore the backup of the SSD to another source like your Mac desktop or another drive. Post back for more instructions.
If you did not do this, Time Machine would not have backed up the SSD.
Is it possible that you might have manually backed up the SSD to another drive or the Time Capsule using the Finder to simply Copy / Paste the data?