The title of the Guardian article is somewhat misleading. The author left out a key word: Western...as in "a 97% decline in (Western) Monarch butterflies". This group is a separate population from the much larger Eastern Monarch population.
The Western Monarch butterfly population that overwinters in California is critically endangered. It was less than 1% of the total Monarch butterfly population to begin with and now has fallen possibly to as low as 30,000 across California (according to the article).
The larger population of Monarchs that overwinter in Mexico has also been devastated. These Monarchs summer in the Midwest, Eastern states, Ontario, etc. I have read elsewhere that it is estimated to be around 93 million, but is thought to have been around a billion at one time. Also, this population is thought to have possibly had a good 2018 in terms of numbers successfully migrating northward last summer.
" To follow the roosts and therefore the migration visit Journey North Monarch Roost Map 2018. While the numbers of reported roosts through 2 September is encouraging, and is a signal of a robust migration, the estimated numbers of monarchs are also higher than seen in several years with 17 different roosts estimated to contain 1000 or more monarchs."