That was the era of lightness uber alles. Tires for racing bikes were narrow and were therefore run at high pressure.

They were also tubulars, which because of their construction are generally more shock absorptive than clinchers/wired-on tires of the same width.

In the pre-Merckx era, when most road races were marathon affairs on dirt and gravel roads, the tires the pros rode on were considerably wider and were ridden at much lower pressures.

Here's an older BikeForums thread that spells some of this out.

Having started racing in mid-'60's myself, it took quite a while for me to accept that wider tires are no slower than narrower. But racing results don't lie.

Or do they? Waiting for someone to respond to Bulgie's "So how is it that Pogacar has been winning Grand Tours on 33-mm tubulars?" question with "If he's winning on 33-mm tires, it's prima facie because he's using better dope than anyone else."