夏 and 冬: summer, winter, and comparison standards
Separate a season's general associations from the information actually stated.
Summer activities
夏 (なつ) means summer. 「夏に海へ行きました」 says the speaker went to the sea in summer.
「今年の夏は去年より涼しかったです」 says this summer was cooler than last summer. Last summer is the comparison standard.
Preparing for winter
冬 (ふゆ) means winter. 「冬の準備を始めました」 means starting preparations for winter.
「冬でも花が咲きます」 says flowers bloom even in winter. でも includes a season that may be unexpected.
Avoid adding seasonal stereotypes
「夏ですが、山の上は寒いです」 says it is summer but cold on the mountain. Summer does not make every location hot.
「冬までに完成する予定です」 sets a planned completion deadline of winter. It does not describe work continuing throughout winter.
Does 「今年の夏は去年より涼しかったです」 necessarily say this summer was cold?
Show the answer and why
No. It compares it as cooler than last summer without declaring it cold in absolute terms.