this advice is specifically about sulfuric acid. it's denser than water, so if added to it it will sink diluting itself along the way, while also heating water around and making it float to the surface. if done opposite way, water won't mix immediately because of large density difference so neutralizatio heat will be deposited on surface between these two boiling water and throwing acid around. this matters less with other acids because less heat is deposited, and in some cases acid is less dense than water. but if you stir the acid quickly, you can do it either way as long as you control temperature. this also is the case when you need to mix two different acids
tldr you can do whatever you want as long as you know what are you doing
e: i've checked and heat of dilution is greatest for sulfuric acid, liquid HF is similar per gram, gaseous HCl and HBr are half of that per mol, other common acids 5-10x less esp as aqueous solutions and not neat. also the same happens when diluting acids with other solvents, like alcohols or ethers, these might be even worse because they boil at lower temperature