You got the facts. Now for the geeks:
The legend about platinum not being resizable is due to the fact that gold is much easier to solder:
1. Typical temperature for gold solder flow is between 700 °C and 900 °C; true platinum solder has a minimum
flow temp of 1300 °C. This is not only harder to achieve; it presents real problems for thin parts and pieces with set gems or in mixed metals (pure gold melts at 1064 °C; alloys usually lower).
2. Gold is relatively tolerant of "dirt"; platinum is not. It won't stick cleanly unless the environment is very clean.
3. Soldering technique on platinum is very different. Flux is often not used (another reason for wanting a very clean environment). Most metal-smithing training focuses on gold and silver, where the techniques are somewhat similar to each other.
4. Many "platinum" solders contain little or no platinum, and this can cause problems for colour matching and polishing (the solder is much softer than the platinum).
5. Laser welding is a common replacement technique for soldering for platinum repairs and alterations precisely because it avoids all of the problems above. But a good laser welder is still costing several $10,000, so not all shops will have or can afford one (never mind the training and expertise to use it properly).
Put all of this together, and you see why the legend arose... but as Jen and DAF have already said, it is perfectly feasible.
For those that want to know more about platinum solders, here
is an interesting article - if slightly difficult to read because of the background; blast amateur site designers. It's about 10 years old, and no doubt things have improved since then, but physics is what it is, and the above problems will remain no matter what the evolution of metallurgy.