I am not surprised that this works fine. Suntour cog spacing varies somewhat between the small and large cogs, but on average it is close to Shimano 6-speed spacing. I am looking at a Suntour and a Shimano Uniglide freewheel right now, and overall it is close - within the tolerances of indexed shifting.

What confuses most folks (including shop guys) about indexed shifting is that they assume that all shifters and derailleurs behave the same. Wrong.... Suntour and Shimano shifters pull different amounts of cable per cog. And the derailleurs behave differently in terms of how much they travel per unit of cable pull. But index-compatible cog spacing is relatively close between the makers. That is, 6-speed freewheels are generally cross compatible, as are 7-speed freewheels. Of course, if you have 6-speed shifters, they are not going to correctly shift a 7-speed freewheel.

To improve your shift performance, now remove the Shimano 600 freewheel with Uniglide cogs, and install a current $15 Shimano Hyperglide freewheel. The Hyperglide cogs, when matched with a current chain, results in much better crisper and faster shifting than Unlglide ever did.