This reticence to verbally express love may be true of Pee and his generation or his own familial background but I wouldn't say it's true of all Thais. My friends are very generous with their verbal outpourings of love during anniversaries, Valentine's Day, special occasions, when they're drunk, when they're trying to make up after a big argument or for no reason other than to express genuine affection. I'm sure it's been said more than a few times during climax too! I see verbal expressions of love on social media, in movies, TV dramas, classical and contemporary literature, hear it in plenty of Thai songs, poetry, etc.
From the formal phom rak khun to the more colloquial chan rak ter, Thais are not shy of saying those universal words I love you.