It doesn't seem to include St Pancras in the itinerary, though?
That screenshot is routing the OP on the Overground from Euston to Willesden Junction, then Overground to Clapham Junction and then from there to Brighton, which would be Southern not Thameslink?
I suspect, then, that the itinerary is displaying "London Thameslink" in error rather than the routing being in error?
I think you're confusing London (terminals) - Brighton,
routed Thameslink only, and London Thameslink [ie Thameslink stations]
origin - Brighton. These are both a similar price (ie cheaper than an any train / any terminal London-Brighton ticket) but have different restrictions.
The Thameslink-only tickets allow any London-Brighton route, using any terminal,
providing you stick to the right colour trains ... which in effect does limit which route and which London terminals you can use of course! The Thameslink origin to Brighton are valid to/from anywhere between St Pancras and London Bridge (also Elephant & C I think) to Brighton. I've had the latter sometimes showing London Thameslink as the origin, and sometimes St Pancras, but they're the same thing here. These are clearly better value than the former type, at the same price, since there's almost nothing the former ticket will do which these won't, but plenty the latter will do that the former won't. So, you can use Thameslink-Brighton tickets on all the valid routes to/from Brighton (eg via Hove or via Lewes, stopping off at such places if wanted), and you can get any colour of train north from Brighton. The only constraint is that you must head for a Thameslink destination - but that covers most places since you can validly travel to London Bridge via Clapham Junction - Waterloo - Waterloo East. The only significant lack of London terminal access with these tickets is Clapham Junction to Victoria, which is off route. (But of course you couldn't do that with the other type of cheap "Thameslink" ticket either, since those trains don't run to Victoria.)
Getting back to the original query here, and to the posting I'm replying to - then if the ticket is a Thameslink (or St P)
origin ticket (not a "
route Thameslink" one) there's no problem using it from Clapham Junction to Brighton, whatever colour of train. The exciting thing about this, though, is the fact that Euston seeming to be (erroneously?) linked to St P as a related station allows one of these tickets to be used on an all-NR route from there via the West London line, as shown. It seems barely worth the bother, given the short distance from Euston to St P and the quicker journey you'd get by walking to St P; and you'd surely never get on to the platform at Euston - but it sounds a fun notion...