Nah, its just a minor limitation of A) the design, and B) fluid trainers in general.
Sure a softer tyre will make slightly more resistance, but we are talking a shortfall of 30W, and Dad isn’t that fit (quite fit for 60+ though). I’m even lighter than he is, and stronger, so more like 70W short on resistance for me.
Generally fluid trainers are designed to give very road like levels of resistance, and if you went out to do big gear intervals on the flat you’d also run out of resistance. For example I use a 2% slope that’s always into a slight headwind in 50-12 or 50-11.
But aside from that 1 minor thing, its been a great trainer, very good ‘feel’ to it and perfectly reliable for 3-4 years so far.