I believe the tweet is not without merit.
The US conducted a study post Seawolf era in determining the appropriate displacement for its next SSN class which today is the Virginia. That study, revolved around three displacement bands and the associated capabilities that could be bundled within each band. The conclusion was that below 6000 tons displacement, it was just insufficiently sized relative to speed and weapons payload requirements. It is no coincidence that the Virginia has a submerged displacement of 7800 tons and the Astute between 7400 to 7800 tons. In comparison, the Barracuda is expected to displace around 5300 tons.
The reason displacement matters because in SSN design, approximately 90 % of the weight is taken up by configuration (hull) and machinery (reactor) leaving less than 10 % for weapons and systems. In other words, the lower the displacement the less relative room is available for weapons and systems. Higher speed requires more associated machinery and hence space It is about trade offs.
Barracuda will probably be very silent but will lack weapons load capacity and we already know it will be non VLS.
