That looks like the High Guard chart on page 53. The stats match previous versions of Traveller
(CT Book 5 - High Guard p.23 for example) and other sources. If it is laziness on Mongoose's part, it's for adhering to established old charts.
A few things I suggest for your consideration.
From a a rules standpoint, the book indicates that overall TL of your "standard spacecraft" is TL12. But is that once a technology is found it is possible in some cases to proto-tech or use primitive versions it as is indicated in High Guard page 52, and prototyping as in Central Supply Catalog. From a rules standpoint that means yes, TL 6 can build standard Manoeuvre Drives :shock: Meh, I go with the flow. Still as the rest of the chart indicates that those same TL6 Manoeuvre Drives would have 200% of the tonnage of the "standard" tech.
With regards to canon, you are always going to have conflicts. GURPS Interstellar Wars indicates that practical reactionless thrusters were invented in 2064 when Terrans were still MgT TL 8. They were not at MgT TL 9 Tech until the 2130's even though they invented Jump-0.25 in 2088 Jump-1 in 2092. But, early Jump Drives were Jump 0.25 and Jump 1 consuming double the amount of fuel normally (5% and 20% tonnage for Jump fuel) found. That same MgT High Guard chart implies you can build Jump 1 with TL 8. Hmm.