It isn't ludicrous in the slightest. It's a measure of comparison, nothing more, nothing less. It doesn't purport to be like for like, it is simply a yardstick. The total value of Russian assets in the UK produces a specific figure. That figure happens to be - if the figure is true in the first place - equivalent to 10% of the UK's annual output. Read into that as much or as little as you like, it's either true or it isn't.
It's like saying the value of my house is 10% of your income. The two are not intended to be directly comparable, but each produces a figure and ti is reasonable to compare the two in absolute terms. People can read into it what they want.
The point about GDP being an inaccurate measure is equally matched by the value of assets being an accurate measure. Property values are guesstimates until the property is sold, likewise yachts businesses and other non-liquid assets. Maybe those values offset inaccuracies in GDP, maybe they don't.
If you don't find the comparison between the two helpful, that's up to you. Doesn't mean one bottom line figure cant be used as a way of conceptualising the other.