If you want bold, use bold. Real bold. Not mathematical symbols. Those characters are intended for use in mathematics only. They are not, semantically, text at all.
U+1D5DB ‹𝗛› \N{MATHEMATICAL SANS-SERIF BOLD CAPITAL H}
U+1D5F2 ‹𝗲› \N{MATHEMATICAL SANS-SERIF BOLD SMALL E}
U+1D5F9 ‹𝗹› \N{MATHEMATICAL SANS-SERIF BOLD SMALL L}
U+1D5FC ‹𝗼› \N{MATHEMATICAL SANS-SERIF BOLD SMALL O}
U+1D43B ‹𝐻› \N{MATHEMATICAL ITALIC CAPITAL H}
U+1D452 ‹𝑒› \N{MATHEMATICAL ITALIC SMALL E}
U+1D459 ‹𝑙› \N{MATHEMATICAL ITALIC SMALL L}
U+1D45C ‹𝑜› \N{MATHEMATICAL ITALIC SMALL O}
Assistive technologies such as screen readers do not interpret these characters as text, but as mathematical symbols, therefore these would be read aloud letter-by-letter as “mathematical bold capital H” or similar. For an example of what this sounds like in practice, see a Tweet from Kent C. Dodds which includes the audio produced by such a screen reader (VoiceOver) when faced with these characters.
The fact that 𝗛 counts as two characters is a separate issue, and is to do with the way that Twitter processes Unicode characters (all Unicode characters, be they text or not). As far as I can tell from the Twitter docs, it should count as one character now, as they count characters (not bytes) in Unicode NFC. However, maybe it was different when you asked the question.