Is this true?

Yes. A "strong background" is a very natural way to describe expertise in a certain field, task, or activity. * I have a strong background in LEGO design and construction. Some of my pieces have won awards. * I have a strong background in building models and miniatures which I have leveraged to build excellent LEGO creations. * My strong background in LEGO building will likely be an asset to the team. * His strong background in all the various types and sizes of LEGO will make a difference. * A strong background is LEGO building is almost mandatory if you expect to win.

Just wanted to add as well, saying that you excel at something would seem boastful in a conversation but is perfectly fine to put in your CV.

It would be better to simply state the prize that you won. That way, you don't need to boast - you're just stating a fact. "I'm very keen on Lego building, and last year I won an award in a competition."

Strong background is definitely a fine way to say that someone has previous experience in an area, but I am more skeptical of the claim it made that using "excel" sounds too much like bragging. I don't think I would take it that way, especially in the context of someone who is supposed to be "selling themselves" so to speak. Like in applications for a job or program, I think saying you "excel" is fine. At least I've always said I excel at all sorts of things on my resumes and cover letters and I never got any kind of feedback from anyone that they thought it sounded too much like I was bragging.