They probably use a bloom filter and populate it with a bunch of 'probably spurious' word pairs. The combination of 'Flower' and 'eggs' will evaluate as 'probably spurious' but the combination 'flower' and 'sugar' evaluates as 'not spurious'. This is probably a manually populated bloom filter of common spurious word pairs.

For reference, a bloom filter is an extremely space efficient, probabilistic data structure that acts a bit like a set and can answer the query 'does the bloom filter contain this entry'. The bloom filter will respond with either 'definitely not' or 'possibly/probably' depending on how it is tuned.

You could conceivably automatically populate this (still hardcoded) bloom filter by doing a brute force language corpus search for heavily correlated word pairs that have one or more of the two words having phonetically similar misspellings. E.g. 'sea' and 'breeze' would be heavily correlated. 'Sea' has a phonetically identical misspelling 'see'. You could then automatically add 'see + breeze' as a spurious pairing to the filter.