I've been in the radio control hobby for almost 20 years and never once considered the proportion of electrolyte to overall weight, so, I can't answer that.
What I **can** tell you - is that LIPOS are far more volatile than most people think, I can point to many articles where they spontaneously explode. In almost all of these cases it is due to the consumer considering them an inanimate and harmless power device and making an error in judgment about charging or using a pack that has been damaged in some way.
An old LIPO or one that has been over-discharged is a primary candidate for ballooning, in which the internal gases overheat and swell the casing, or combustion. When these babies go, they go with full force - explosions that send the entire pack across the room, and they burn at over 1000 F.
Most of these events are in the hobby, where the packs are charged at the fastest charge rate possible and the user's try to get the most out of the packs, but there have been incidents of exploding cell phones and laptops. In most cases, such as laptops, the LIPOS are charged with "slow chargers" but when the pack begins to age even these can create problems.
So this restriction probably is very warranted, although I don't know why a volatile pack inside a laptop is any less prone to combustion than one in the cargo hold unless it's the fact that the carry-on is at least partially monitored and the one in the cargo hold is not. LIPOS will usually precede their ending with a few minutes of popping, fizzing, and obnoxious smoking before they go.