I've had just the opposite experience with similar sized aluminum frames. A 1997 CAD3 and a 2021 Trek ALR 5. Both bikes are close in geometry, with the ALR being slightly more relaxed. The old bike ran 23mm and the new bike came with 25mm. I keep my tire pressures high.
The ALR 5 is worlds smoother than the CAD3. The ALR is stiff in terms of power delivery, but the road buzz and harshness are just eliminated. Differences in the geometry are understood - but the bikes are worlds apart.
I also had an old steel Trek 330 and currently have a steel 2001 Lemond Zurich. Both older steel frames - night and day difference. Both are smooth, limited road buzz... the 330 was like riding a wet noodle. Frame flex, BB flex, no real feel to the bike - I could make the chainring rub the front mech under heavy loads, the bike was a noodle. The Lemond is smooth and stiff at the same time, the bike is worlds better.
I also have a CF cyclocross frame, a Giant TCX - the bike, even on 37mm tires with a flexy seat post, is brutal. Stiff, harsh, road buzz...