Is it normal that journals strongly suggest a professional English editing service?
In one journal where I am familiar with the editorial workflow, the review form explicitly asks reviewers to rate the language quality of the manuscript. Based on this rating, the editor can tick an item similar to "Needs language revisions" when putting together the decision letter. The decision letter will then contain a paragraph that advises the authors to do a language revision. I am not sure at the moment whether a particular service is being recommended there.

The point is that such a recommendation can get into the letter easily, but will not be included by default. Probably the editor only wrote the first two or three sentences of the letter, maybe without thinking carefully about the exact formulation, and the rest is based on a customizable template. Nevertheless, it usually means that at least one reviewer was criticizing language usage, maybe even without giving specific comments on it. I would advise you to at least double-check on language usage, and if possible have it proof-read by someone else with very good English skills or a native speaker.

However, as long as the reviewers can understand the technical content well, these points are usually not decisive for the acceptance of the manuscript. Especially, as long as any language problems are corrected, I can't imagine that the editor will care whether you use the suggested language service or not.