"Compacted for ESL" means the FormID of a mod's plugin have been set to the range for ESL. It does NOT mean the file extension was changed to "ESL". Maybe the extension was changed to ESL to force a high load order or maybe it was kept ESP to make installation easier and overwrite an existing ESP of the same file name.
An ESL file is just a plugin with the .ESL file name extension. They load high and can be good for avoiding worldspace conflicts.
Some of my Power Armor files are ESL flagged ESP. That IS an "ESLified" file. That file name still has the .ESP extension. They do not count against the ESM+ESP plugin limit but do allow moving the file in load order.
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