Since I looked this up elsewhere, these were the availabilities left on this return service at ~ around the time of the original Twitter post (half an hour before):
You can see exactly what's happened here. No availability in standard class en-route, only a handful of very expensive walk-up first class fares. No splits available - standard class appeared to be completely full up. Young people are disproportionately affected too, because they're not able to use their railcard on walk-up first class tickets. The total price on Trainline is likely a combination of a Standard class Advance for the outward (which may be split somewhere, and is probably railcard-discountable), and one of the tickets above.
The mandatory reservations flag then means that instead of flogging the walk-up prices in standard that have previously been identified at the start of this thread:
customers are instead mis-sold these eye-wateringly expensive fares (when they frankly don't want to travel in 1st), which are the only ones a compliant TIS is
allowed to sell before simply showing a service as "Sold Out". Meanwhile if they go to a ticket office they'll pay £44.20 with their 16-25 railcard.